Q160. In a concave mirror, a ray is incident parallel to the principal axis. After reflection, the ray passes through the _______.
[RRB Group D, 21 Aug 2026]
(A) Anywhere on the mirror
(B) Pole
(C) Focus
(D) Centre of curvature
Answer: (C) Focus
Explanation: A concave mirror is converging: rays parallel to its principal axis reflect through the principal focus. This occurs because the curved reflecting surface changes their directions according to the law of reflection. The focus lies in front of the mirror.
Q159. Which scenario best illustrates the use of a convex mirror based on the nature of its image?
[RRB Group D, 20 Aug 2026]
(A) A model uses it as a makeup mirror
(B) A scientist uses it to focus sunlight for heating
(C) A student uses it to obtain a real, inverted image on paper
(D) A shopkeeper uses it to monitor a wide area inside the store
Answer: (D) A shopkeeper uses it to monitor a wide area inside the store
Explanation: A convex mirror gives a wide field of view by forming virtual, upright and diminished images. Therefore, it is useful for surveillance inside shops and as a vehicle rear-view mirror. It cannot concentrate sunlight or produce screen images.
Q158. An object is placed between the optical centre and principal focus of a convex lens. What is the nature of its image?
[RRB Group D, 13 Aug 2026]
(A) Real, inverted and diminished
(B) Real, inverted and enlarged
(C) Virtual, inverted and diminished
(D) Virtual, erect and magnified
Answer: (D) Virtual, erect and magnified
Explanation: For an object inside the focal distance of a convex lens, emerging rays diverge and their backward extensions meet on the object’s side. This creates an upright, enlarged virtual image, which explains the working of a simple magnifying glass.
Q157. What is the angle between the incident and emergent rays called in the context of a prism?
[RRB Group D, 11 Aug 2026, Shift 2]
(A) The angle of incidence
(B) The angle of refraction
(C) The angle of emergence
(D) The angle of deviation
Answer: (D) The angle of deviation
Explanation: Light refracts at both faces of a prism, changing its overall direction. The angular difference between the original direction of the incident ray and the final emergent ray is the angle of deviation, not either individual refraction angle.
Q156. The refractive index of water relative to air is the ratio of which speeds?
[RRB Group D, 11 Aug 2026, Shift 3]
(A) Sum of speeds in air and water
(B) Difference of speeds in air and water
(C) Speed in water divided by speed in air
(D) Speed in air divided by speed in water
Answer: (D) Speed in air divided by speed in water
Explanation: Relative refractive index n(water/air) equals v(air)/v(water). Light slows as it enters water from air, so the relative index is greater than one. The ratio is dimensionless because both speeds have identical units.
Q155. Which image is formed by a concave mirror when the object lies between its focal point and centre of curvature?
[RRB Group D, 10 Aug 2026]
(A) Real, inverted and magnified
(B) Virtual, upright and diminished
(C) Real, upright and same size
(D) Virtual, inverted and magnified
Answer: (A) Real, inverted and magnified
Explanation: An object between F and C produces reflected rays meeting beyond C. Because the rays physically intersect, the image is real; its orientation is inverted and its height exceeds the object’s. This is the magnifying real-image configuration of concave mirrors.
Q154. Which application uses refraction to focus light onto a specific point for clear vision?
[RRB Group D, 9 Aug 2026]
(A) The human eye lens adjusting its shape
(B) A glass prism splitting sunlight
(C) A plane mirror reflecting images
(D) A shadow formed behind an object
Answer: (A) The human eye lens adjusting its shape
Explanation: The eye lens refracts entering light and focuses it on the retina. Ciliary muscles vary the lens curvature and therefore its optical power, enabling clear sight at changing distances. This change in focusing ability is called accommodation.
Q153. The refracted ray inside a triangular prism bends ________.
[RRB Group D, 9 Aug 2026]
(A) Perpendicular to the base
(B) Toward the base of the prism
(C) Along the base
(D) Away from the base
Answer: (B) Toward the base of the prism
Explanation: Refraction at a prism’s surfaces results in a net deviation toward its base. The ray bends toward the normal on entering glass and away from the normal on leaving it. Their geometry produces overall deviation from the original course.
Q152. What is the angle between the two refracting faces of a triangular glass prism called?
[RRB Group D, 6 Aug 2026]
(A) Angle of refraction
(B) Angle of deviation
(C) Angle of the prism
(D) Angle of incidence
Answer: (C) Angle of the prism
Explanation: The angle formed where the two refracting faces meet is the prism angle, usually represented by A. It is a fixed geometric property of the prism, whereas angles of incidence, emergence and deviation depend on the light’s path.
Q151. Why does a pencil partially immersed in water appear bent at the surface?
[RRB Group D, 6 Aug 2026]
(A) Reflection from the pencil
(B) The pencil emits light at another angle
(C) Light changes direction between water and air
(D) Water physically changes the pencil’s thickness
Answer: (C) Light changes direction between water and air
Explanation: Rays coming from the submerged pencil refract when travelling from water into air, bending away from the normal. The eye traces them backward in straight lines and sees the submerged part at a shifted apparent position, making the pencil appear bent.
Q150. Which law states that the incident ray, refracted ray and normal at the interface lie in the same plane?
[RRB Group D, 5 Aug 2026]
(A) Law of reflection only
(B) Law of dispersion
(C) Second law of refraction
(D) First law of refraction
Answer: (D) First law of refraction
Explanation: The first law of refraction states that the incident ray, refracted ray and the normal at the point of incidence are coplanar. Snell’s law, normally called the second law, relates the sines of their angles through refractive indices.
Q149. A swimming pool appears shallower than its actual depth owing to which phenomenon?
[RRB Group D, 3 Aug 2026]
(A) Refraction
(B) Diffraction
(C) Dispersion
(D) Interference
Answer: (A) Refraction
Explanation: Light from the pool floor bends away from the normal upon emerging from water into air. The observer extends the outgoing ray backward, locating the bottom at a smaller apparent depth. Therefore, the pool looks shallower than it is.
Q148. Who is popularly known as the father of fibre optics?
[SSC GD Constable, 7 May 2026]
(A) C. V. Raman
(B) Homi J. Bhabha
(C) Narinder Singh Kapany
(D) Satyendra Nath Bose
Answer: (C) Narinder Singh Kapany
Explanation: Narinder Singh Kapany is widely called the father of fibre optics for pioneering work in transmitting images and light through bundles of glass fibres. Optical fibres use total internal reflection to confine and guide light, forming the basis of modern optical communication.
Q147. Where does a convex lens bring rays travelling parallel to its principal axis to a focus?
[RRB Group D, 10 Feb 2026]
(A) At its optical centre
(B) At its near-side focus
(C) At its principal focus on the opposite side
(D) At twice its focal length
Answer: (C) At its principal focus on the opposite side
Explanation: A convex or converging lens redirects paraxial rays parallel to the principal axis so that they meet at its principal focus on the other side. This point determines the focal length and underlies lens image formation.
Q146. An object is 80 cm in front of a convex lens of focal length 40 cm. How far away will the real image form?
[RRB Group D, 10 Feb 2026]
(A) 80 cm
(B) 40 cm
(C) 20 cm
(D) 160 cm
Answer: (A) 80 cm
Explanation: The object is at twice the focal length, 2f, of the convex lens. Applying 1/f = 1/v − 1/u gives u = −80 cm and v = +80 cm. The resulting image is real, inverted and equal-sized.
Q145. Which phenomenon is NOT an example of refraction of light?
[RRB Group D, 9 Feb 2026]
(A) Apparent bending of a pencil in water
(B) Reddish appearance of the setting Sun
(C) Shifting apparent depth in water
(D) Atmospheric bending of starlight
Answer: (B) Reddish appearance of the setting Sun
Explanation: A red sunset is primarily explained by wavelength-dependent scattering, not refraction. During the long atmospheric path at sunset, shorter blue wavelengths scatter out of direct sunlight more strongly, leaving proportionally more red and orange light to reach the observer.
Q144. Why do distant objects seem to waver when seen through air over a hot road?
[RRB Group D, 9 Feb 2026]
(A) Diffraction by dust particles
(B) Total internal reflection in glass
(C) Polarisation by clouds
(D) Atmospheric refraction through variable-temperature air
Answer: (D) Atmospheric refraction through variable-temperature air
Explanation: Air above hot surfaces has fluctuating temperature and density, so its refractive index continually changes. Light rays passing through these layers bend by varying amounts, causing shimmering, wavering and distorted views of distant objects. This is a form of atmospheric refraction.
Q143. What type of image does a concave lens form for a real object?
[RRB Group D, 9 Feb 2026]
(A) Always real and inverted
(B) Always magnified and real
(C) Always virtual, erect and diminished
(D) Always real and equal-sized
Answer: (C) Always virtual, erect and diminished
Explanation: A concave lens diverges incident rays. The backward extensions of emergent rays appear to meet between its optical centre and principal focus on the object’s side. Thus, for an ordinary real object, the image is virtual, upright and smaller.
Q142. Which statement explains why a concave lens cannot form a real image on a screen from a real object?
[RRB Group D, 9 Feb 2026]
(A) It has no optical centre
(B) It is an opaque material
(C) It produces only reflected rays
(D) Its refracted rays diverge and form a virtual image
Answer: (D) Its refracted rays diverge and form a virtual image
Explanation: A concave lens spreads incoming rays apart. A real screen image requires rays actually to converge and intersect, which does not happen for a real object in front of a lone concave lens. Its image exists only at an apparent intersection.
Q141. What is the magnitude of focal length of a concave lens of power −4 dioptres?
[RRB Group D, 6 Feb 2026]
(A) 4 cm
(B) −4 cm
(C) 40 cm
(D) 25 cm
Answer: (D) 25 cm
Explanation: The reciprocal relation P = 1/f uses metres. With P = −4 D, f = −0.25 m or −25 cm. Its magnitude is 25 cm; the minus sign indicates the diverging nature of a concave lens.
Q140. Which statements about a concave lens are true:
(i) it diverges rays;
(ii) it always produces real images of real objects;
(iii) it produces erect virtual images?
[RRB Group D, 6 Feb 2026]
(A) Only (i)
(B) Only (ii) and (iii)
(C) Only (i) and (iii)
(D) All three
Answer: (C) Only (i) and (iii)
Explanation: A concave lens is diverging, so statement (i) holds. For a real object, it forms a virtual, erect image on the same side as the object, making statement (iii) true. Such a lens alone does not yield a real image.
Q139. What is the correct sequence of colours in the spectrum obtained from white light through a prism?
[RRB Group D, 5 Feb 2026]
(A) ROYGBIV only
(B) VIBGYOR in random order
(C) Violet, blue, yellow, orange, indigo, green, red
(D) Violet, indigo, blue, green, yellow, orange, red
Answer: (D) Violet, indigo, blue, green, yellow, orange, red
Explanation: The visible spectrum is conventionally recalled as VIBGYOR: violet, indigo, blue, green, yellow, orange and red. Different wavelengths refract by different amounts within glass; violet deviates most and red least, causing white light to separate.
Q138. An object is at u = −30 cm and its image is at v = +60 cm for a lens. What is the magnification?
[RRB Group D, 5 Feb 2026]
(A) −2
(B) +2
(C) −0.5
(D) +0.5
Answer: (A) −2
Explanation: For a thin lens, linear magnification m equals v/u. Substituting +60 cm and −30 cm gives m = −2. Its magnitude indicates a doubled image height, while the negative sign signifies that the real image is inverted.
Q137. Under the Cartesian sign convention, what is the sign of the focal length of a concave mirror?
[RRB Group D, 5 Feb 2026]
(A) Positive always
(B) Negative always
(C) Zero always
(D) Depends solely on object height
Answer: (B) Negative always
Explanation: For mirrors, distances measured opposite the incident light direction are negative. A concave mirror’s principal focus lies in front of the reflecting surface, on the incoming-light side. Its focal length is consequently negative under the standard Cartesian convention.
Q136. Why does a soap solution sometimes appear cloudy and show a path of light?
[RRB Group D, 5 Feb 2026]
(A) Soap molecules emit light
(B) Soap undergoes total absorption
(C) The soap particles reflect sunlight like mirrors only
(D) Colloidal micelles scatter light (Tyndall effect)
Answer: (D) Colloidal micelles scatter light (Tyndall effect)
Explanation: Soap solutions can contain colloidal micelles large enough to scatter light. The scattered radiation makes a light beam’s path visible, known as the Tyndall effect. The micelles do not need to emit light; they redirect incident light.
Q135. The absolute refractive index of an ordinary transparent medium is usually _______.
[RRB Group D, 4 Feb 2026]
(A) Greater than one
(B) Negative
(C) Exactly zero
(D) Less than zero
Answer: (A) Greater than one
Explanation: Absolute refractive index is defined as n = c/v, where c is vacuum light speed and v is light speed in the material. Since ordinary transparent media transmit light more slowly than vacuum, their refractive indices exceed unity.
Q134. What phenomenon makes a beam of light visible when it passes through a fine colloidal suspension?
[RRB Group D, 4 Feb 2026]
(A) Polarisation
(B) Diffraction
(C) Tyndall effect
(D) Regular reflection
Answer: (C) Tyndall effect
Explanation: The Tyndall effect is the scattering of light by sufficiently small dispersed particles in a colloid. Because some light is redirected sideways toward the viewer, the beam itself becomes visible in an otherwise transparent-looking suspension.
Q133. Which feature characterises a starch colloid illuminated by a strong beam?
[RRB Group D, 4 Feb 2026]
(A) It scatters light while its dispersed particles do not settle rapidly
(B) It never scatters any light
(C) Its particles always settle immediately
(D) It acts as a perfect plane mirror
Answer: (A) It scatters light while its dispersed particles do not settle rapidly
Explanation: Starch dispersed as a colloid consists of particles capable of scattering light and showing the Tyndall effect. Their small size and interactions with the medium prevent rapid settling, unlike the larger particles of coarse suspensions.
Q132. Where is the image of a very distant object formed by a convex lens?
[RRB Group D, 4 Feb 2026]
(A) At 2F
(B) At the principal focus
(C) At the optical centre
(D) At infinity
Answer: (B) At the principal focus
Explanation: A distant object’s light reaches a lens as approximately parallel rays. A convex lens converges them at its far-side principal focus. Consequently, the image is highly diminished, usually described as point-sized when the source is very distant.
Q131. Where is the centre of curvature of a convex mirror located?
[RRB Group D, 3 Feb 2026]
(A) In front of the mirror
(B) At its pole
(C) At infinity
(D) Behind the reflecting surface
Answer: (D) Behind the reflecting surface
Explanation: A convex mirror bulges toward incident light. Its centre of curvature belongs to the imaginary sphere of which the mirror is a segment, and lies behind the reflecting face. Hence its principal focus is virtual and behind the mirror.
Q130. Which statements about light scattering are incorrect:
(i) it is independent of particle size;
(ii) it is directly proportional to wavelength?
[RRB Group D, 3 Feb 2026]
(A) Only (i)
(B) Both (i) and (ii)
(C) Only (ii)
(D) Neither (i) nor (ii)
Answer: (B) Both (i) and (ii)
Explanation: Scattering depends strongly on particle size relative to the light wavelength. For sufficiently small particles, Rayleigh scattering varies inversely as the fourth power of wavelength, so shorter wavelengths scatter more. Both quoted statements are therefore incorrect.
Q129. Which claims about the Cartesian sign convention for lenses are false:
(i) distances originate at focus;
(iii) heights above axis are negative?
[RRB Group D, 2 Feb 2026]
(A) Both (i) and (iii)
(B) Only (i)
(C) Only (iii)
(D) Neither
Answer: (A) Both (i) and (iii)
Explanation: Lens distances are measured from the optical centre, not a principal focus, so statement (i) is false. Heights above the principal axis count as positive, whereas those below are negative; therefore statement (iii) is also false.
Q128. Where should an object be placed before a convex lens to form a real image of exactly the same size?
[RRB Group D, 9 Jan 2026]
(A) At F
(B) At 2F
(C) Beyond 2F
(D) Between lens and F
Answer: (B) At 2F
Explanation: An object at twice the focal length of a convex lens has its image at 2F on the other side. The lens equation yields equal magnitudes of object and image distances, so the magnification is −1.
Q127. What is meant by one dioptre of lens power?
[RRB Group D, 9 Jan 2026]
(A) A lens with 1 cm focal length
(B) The power of a lens of focal length 1 metre
(C) A focal length of 10 metres
(D) The brightness of a light source
Answer: (B) The power of a lens of focal length 1 metre
Explanation: One dioptre is the optical power of a lens with focal length one metre. Power equals 1/f when f is measured in metres. Positive dioptres signify converging lenses and negative values signify diverging lenses.
Q126. What is the relation between radius of curvature R and focal length f of a spherical mirror?
[RRB Group D, 29 Dec 2025]
(A) R = f
(B) R = f/2
(C) R = 2f
(D) R = 4f
Answer: (C) R = 2f
Explanation: For a small-aperture spherical mirror, the principal focus lies midway between pole and centre of curvature. Therefore f = R/2, or R = 2f, with appropriate Cartesian signs. The relation applies to both concave and convex spherical mirrors.
Q125. Where do rays parallel to the principal axis meet after passing through a convex lens?
[RRB Group D, 27 Dec 2025]
(A) At the principal focus
(B) At twice the focus
(C) At the optical centre
(D) At the first surface
Answer: (A) At the principal focus
Explanation: A convex lens is thicker at its middle and ordinarily acts as a converging lens in air. Rays incident parallel to the principal axis are refracted toward the axis and meet at the far-side principal focus, enabling real image formation.
Q124. Which mirror should be used for shaving to obtain an enlarged upright image of the face?
[RRB Group D, 24 Dec 2025]
(A) Convex mirror
(B) Concave mirror
(C) Plane mirror
(D) Two-way mirror
Answer: (B) Concave mirror
Explanation: When a face is positioned between a concave mirror’s pole and focal point, the reflected rays diverge but appear to meet behind the mirror. The image is virtual, upright and larger, making the mirror useful for shaving.
Q123. Where is the image formed if an object is kept at the focus of a convex lens?
[RRB Group D, 23 Dec 2025]
(A) At its optical centre
(B) At infinity
(C) At twice the focal length
(D) On the same side between F and O
Answer: (B) At infinity
Explanation: Placing an object at the first principal focus of a convex lens produces emerging rays parallel to each other. Because they do not meet at a finite distance, the image is described as real, inverted, enlarged and located at infinity.
Q122. What is the sign of the focal length of a concave lens according to the Cartesian convention?
[RRB Group D, 23 Dec 2025]
(A) Positive
(B) Zero
(C) Sometimes infinite
(D) Negative
Answer: (D) Negative
Explanation: The principal focus of a concave lens lies on the incident-light or object side. Under the standard Cartesian sign convention, distances toward incoming light are negative. Thus a concave lens has negative focal length and negative optical power.
Q121. Which statement correctly defines the power of a thin lens?
[RRB Group D, 21 Dec 2025]
(A) Product of focal length and aperture
(B) Reciprocal of focal length in metres
(C) Distance between optical centre and object
(D) Ratio of image to object size
Answer: (B) Reciprocal of focal length in metres
Explanation: Lens power measures how strongly a lens converges or diverges light. Numerically P = 1/f, where focal length is in metres and power in dioptres. A shorter absolute focal length implies greater refracting ability.
Q120. Why does a dentist use a concave mirror when examining a patient’s teeth?
[RRB Group D, 19 Dec 2025]
(A) To reduce the image
(B) To obtain an enlarged, upright image at close distance
(C) To see a wide field of view with a smaller image
(D) To split white light into colours
Answer: (B) To obtain an enlarged, upright image at close distance
Explanation: A dentist holds the concave mirror so that the tooth is within its focal length. In this position the mirror forms a virtual, erect, enlarged image, helping the dentist inspect small structures more clearly.
Q119. Which statement about a lens of power +5 D is incorrect?
[RRB Group D, 19 Dec 2025]
(A) It is converging
(B) Its focal length is 20 metres
(C) Its focal length is 0.20 metre
(D) Its focal length is 20 centimetres
Answer: (B) Its focal length is 20 metres
Explanation: Using P = 1/f, a +5-dioptre lens has f = +0.20 metre or +20 centimetres, not 20 metres. The positive power indicates a converging lens. Power and focal length have reciprocal numerical values only when metres are used.
Q118. What is the focal length of a convex mirror of radius of curvature 4 m?
[RRB Group D, 19 Dec 2025]
(A) 2 m
(B) 4 m
(C) 8 m
(D) 1 m
Answer: (A) 2 m
Explanation: For any paraxial spherical mirror, focal length is half its radius of curvature. Thus R = 4 m gives f = 2 m. In the Cartesian convention, a convex mirror’s focal length is positive because its focus lies behind the mirror.
Q117. A spherical mirror reflects light from a surface bulging outward, with its centre of curvature behind it. Name the mirror.
[RRB Group D, 17 Dec 2025]
(A) Plane mirror
(B) Concave mirror
(C) Parabolic concave mirror
(D) Convex mirror
Answer: (D) Convex mirror
Explanation: The outer surface of a convex spherical mirror is reflective and bulges toward the observer. Its centre of curvature and virtual principal focus lie behind the reflecting surface. This structure creates reduced, upright images and an extensive field of view.
Q116. What image does a convex mirror form of an object placed at a finite distance?
[RRB Group D, 16 Dec 2025]
(A) Real and inverted
(B) Virtual, erect and magnified
(C) Virtual, erect and diminished
(D) Real and magnified
Answer: (C) Virtual, erect and diminished
Explanation: Reflected rays from a convex mirror diverge, appearing to originate from behind its surface. For every real object at finite distance, the image is virtual, upright and diminished, positioned between the pole and the virtual focus.
Q115. Which phenomenon is NOT responsible for the image made by a convex mirror?
[RRB Group D, 11 Dec 2025]
(A) Reflection
(B) Refraction
(C) Following the reflection laws
(D) Divergence of reflected rays
Answer: (B) Refraction
Explanation: A convex mirror creates an image through reflection, not by transmitting and refracting light. Its outwardly curved surface reflects light into divergent paths whose backward extensions appear to meet behind the mirror, generating a virtual image.
Q114. A plane mirror forms an image at what distance from its surface compared with the object’s distance?
[RRB Group D, 11 Dec 2025]
(A) Equal distance behind the mirror
(B) Twice the distance behind it
(C) Half the distance behind it
(D) At the mirror surface
Answer: (A) Equal distance behind the mirror
Explanation: In a plane mirror, equal angles of incidence and reflection produce reflected rays whose extensions converge apparently behind the glass. The virtual image appears the same perpendicular distance behind the mirror as the object is in front.
Q113. Which image does a concave mirror form when the object lies beyond its centre of curvature?
[RRB Group D, 10 Dec 2025]
(A) Real, inverted and diminished
(B) Virtual and enlarged
(C) Real, erect and diminished
(D) Virtual and equal-sized
Answer: (A) Real, inverted and diminished
Explanation: When an object is beyond C of a concave mirror, the reflected rays cross between F and C. Their actual intersection makes the image real; it is inverted and smaller because the image distance is less than the object distance.
Q112. A light ray passing through the optical centre of a thin lens is generally ________.
[RRB Group D, 10 Dec 2025]
(A) Undeviated
(B) Totally reflected
(C) Refracted through 90 degrees
(D) Dispersed into seven colours
Answer: (A) Undeviated
Explanation: In the thin-lens approximation, a ray passing through the optical centre emerges in essentially the same direction. This construction rule simplifies ray diagrams and works for both converging and diverging lenses when lens thickness is negligible.
Q111. Which formula expresses the relationship between object distance u, image distance v and focal length f for a thin lens?
[RRB Group D, 9 Dec 2025]
(A) 1/f = 1/u + 1/v
(B) 1/f = u + v
(C) 1/f = 1/v − 1/u
(D) f = uv
Answer: (C) 1/f = 1/v − 1/u
Explanation: Under the Cartesian sign convention, the thin-lens equation is 1/f = 1/v − 1/u. With the correct distance signs, it predicts the position of real or virtual images produced by convex and concave lenses.
Q110. Two separated convex lenses are arranged for minimum chromatic and spherical aberrations. What is f1:f2?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) 4:1
(B) 2:3
(C) 2:1
(D) 3:1
Answer: (D) 3:1
Explanation: For an appropriately separated pair, zero chromatic aberration requires d = (f1 + f2)/2, while minimum spherical aberration requires d = f1 − f2. Equating these conditions gives f1 = 3f2 and hence a 3:1 ratio.
Q109. How many components does a normal Zeeman spectral line split into?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) Three
(B) Two
(C) Four
(D) Five
Answer: (A) Three
Explanation: In the normal Zeeman effect, an atomic spectral line splits into three components in an external magnetic field. The components correspond to allowed changes in magnetic quantum number of −1, 0 and +1. This demonstrates the magnetic influence on emission frequencies.
Q108. What is the area of the first Fresnel half-period zone on a spherical wavefront?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) 2πbλ
(B) πbλ/2
(C) πb²λ
(D) 4πb/λ
Answer: (A) 2πbλ
Explanation: Successive Fresnel half-period zones contribute wavelets of approximately alternating phase to a point of observation. In the usual spherical-wave construction, the first zone area is expressed as 2πbλ under the problem’s geometry and definitions.
Q107. A quarter-wave plate combined with a Nicol prism can produce which polarisation state?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) Unpolarised light
(B) Circularly polarised light
(C) Only longitudinal waves
(D) Only incoherent light
Answer: (B) Circularly polarised light
Explanation: A Nicol prism produces linearly polarised light. A quarter-wave plate introduces a 90-degree phase difference between perpendicular field components. When the input makes the appropriate 45-degree angle with its axes, the transmitted light becomes circularly polarised.
Q106. Arrange orange (IV), yellow (III), green (II) and blue (I) by increasing deviation through a glass prism.
[UPPSC LT Grade Hindi, 6 Dec 2025]
(A) I, II, III, IV
(B) II, III, IV, I
(C) IV, III, II, I
(D) III, I, IV, II
Answer: (C) IV, III, II, I
Explanation: Redder colours deviate less than bluer colours in ordinary glass because refractive index increases as visible wavelength decreases. Thus among orange, yellow, green and blue, the sequence of increasing prism deviation is orange, yellow, green, then blue.
Q105. Which structure provides fine adjustment of the focal length in the human eye?
[RRB Group D, 5 Dec 2025]
(A) Cornea alone
(B) Iris
(C) Optic nerve
(D) Crystalline eye lens
Answer: (D) Crystalline eye lens
Explanation: The crystalline eye lens adjusts its curvature through the action of ciliary muscles and suspensory structures. This changes optical power to keep objects at different distances in focus on the retina. The iris instead regulates incoming light.
Q104. Under the Cartesian sign convention, distances for a lens are measured from its ________.
[RRB Group D, 4 Dec 2025]
(A) Principal focus
(B) Centre of curvature
(C) Optical centre
(D) Outer rim
Answer: (C) Optical centre
Explanation: The optical centre is the reference point for measuring object and image distances in lens equations. Distances along incident light are positive, opposite distances negative. Measurements should not begin at either focus, which is a separate optical point.
Q103. A concave mirror produces what image if the object lies between its pole and principal focus?
[RRB Group D, 3 Dec 2025]
(A) Real, inverted and enlarged
(B) Real, inverted and reduced
(C) Virtual, erect and enlarged
(D) Virtual, erect and diminished
Answer: (C) Virtual, erect and enlarged
Explanation: If an object lies closer to a concave mirror than its focal point, reflected rays diverge and their extensions meet behind the mirror. This gives an upright magnified virtual image, the condition used in shaving mirrors.
Q102. Which statement about a spherical lens is incorrect?
[RRB Group D, 2 Dec 2025]
(A) It can have a principal axis
(B) It has an optical centre
(C) Its optical centre always coincides with its centre of curvature
(D) It can refract light
Answer: (C) Its optical centre always coincides with its centre of curvature
Explanation: A lens has an optical centre through which a paraxial ray passes nearly undeviated, while its curved surfaces have separate centres of curvature. These locations do not generally coincide. The distinction matters when drawing accurate lens geometry.
Q101. What happens when light passes obliquely through a parallel-sided glass slab?
[RRB Group D, 28 Nov 2025]
(A) It travels faster in glass and emerges perpendicular
(B) It slows in glass and emerges at an unrelated angle
(C) Its speed remains unchanged and it reflects totally
(D) It slows within glass and emerges parallel to the incident ray
Answer: (D) It slows within glass and emerges parallel to the incident ray
Explanation: Light slows and bends toward the normal upon entering glass. At the second parallel surface, it speeds up and bends away from the normal by an equal complementary amount, emerging parallel to the incident ray but laterally displaced.
Q100. Why do printed letters below a thick glass slab appear raised?
[RRB Group D, 27 Nov 2025]
(A) Refraction at the slab surfaces
(B) Diffraction of printed ink
(C) Polarisation of reflected light
(D) Absorption by glass
Answer: (A) Refraction at the slab surfaces
Explanation: Light travelling from glass into air bends away from the normal. Observers trace its outgoing rays back along straight lines, causing the letters to appear closer to the upper surface. This optical shift reduces apparent depth.
Q99. Which phenomenon makes light bend around the edges of a small obstruction?
[SSC CGL, 23 Sep 2024]
(A) Reflection
(B) Diffraction
(C) Dispersion
(D) Refraction
Answer: (B) Diffraction
Explanation: Diffraction is the spreading or bending of waves at apertures and around obstacles, especially when their dimensions are comparable with the wavelength. It is evidence of the wave nature of light and differs from refraction at a medium boundary.
Q98. Who formulated the mathematical relationship commonly known as Snell’s law of refraction?
[SSC CGL, 23 Sep 2024]
(A) Isaac Newton
(B) Christiaan Huygens
(C) Willebrord Snellius
(D) Michael Faraday
Answer: (C) Willebrord Snellius
Explanation: Snell’s law relates angles of incidence and refraction through n1 sin i = n2 sin r. The law is named for the Dutch mathematician Willebrord Snellius, although the relationship has an earlier history in several scientific traditions.
Q97. Where does a convex lens form the image of an object placed at its principal focus?
[SSC CGL, 13 Sep 2024]
(A) At the optical centre
(B) At 2F
(C) At infinity
(D) Between F and 2F
Answer: (C) At infinity
Explanation: When the object is at the focal point of a convex lens, refracted rays leave parallel to one another. Parallel rays intersect only at infinity in geometrical optics, so the image is described as highly enlarged and infinitely distant.
Q96. What is the unit of power of a lens?
[SSC CGL, 11 Sep 2024]
(A) Lumen
(B) Candela
(C) Watt
(D) Dioptre
Answer: (D) Dioptre
Explanation: Lens power equals the reciprocal of focal length expressed in metres. Its unit is the dioptre, symbol D, equivalent to one inverse metre. Lumen and candela measure photometric quantities, while watt measures energy transferred per second.
Q95. Who developed the wave theory of light explaining reflection and refraction?
[SSC CHSL, 11 Jul 2024]
(A) Isaac Newton
(B) Thomas Edison
(C) Christiaan Huygens
(D) J. J. Thomson
Answer: (C) Christiaan Huygens
Explanation: Christiaan Huygens developed a wave description in which every point on a wavefront acts as a source of secondary wavelets. Huygens’ principle helps explain the construction of reflected and refracted wavefronts and supports the wave model of light.
Q94. Which statements correctly describe the formation of a primary rainbow?
[UPSC NDA, 21 Apr 2024]
(A) Only reflection, with no refraction
(B) Refraction and one internal reflection inside droplets
(C) Only diffraction of sunlight
(D) Refraction and two internal reflections inside droplets
Answer: (B) Refraction and one internal reflection inside droplets
Explanation: A primary rainbow is produced when sunlight refracts entering a raindrop, reflects once internally, and refracts again upon emerging. Dispersion separates wavelengths into an arc with red on the outside and violet toward the inside.
Q93. Which of these is NOT a component of a compound microscope?
[UPSC NDA, 21 Apr 2024]
(A) Objective lens
(B) Eyepiece lens
(C) Body tube
(D) Retina
Answer: (D) Retina
Explanation: A compound microscope contains an objective lens and an eyepiece lens mounted along an optical tube. The retina is part of the observer’s eye, not the instrument itself. The objective forms an intermediate image magnified by the eyepiece.
Q92. The objective and eyepiece of a standard compound microscope are typically which kind of lenses?
[UPSC NDA, 21 Apr 2024]
(A) Two convex lenses
(B) Two concave lenses
(C) A concave and a plane mirror
(D) Two plane glass plates
Answer: (A) Two convex lenses
Explanation: A conventional compound microscope uses two positive or convex lenses. The objective forms a real, enlarged intermediate image near the eyepiece focal plane, which the eyepiece views as a magnifier to produce a larger virtual image.
Q91. What is the characteristic colour of light emitted by a ruby laser?
[UPPSC RO/ARO, 11 Feb 2024]
(A) Green
(B) Blue
(C) Red
(D) Yellow
Answer: (C) Red
Explanation: A ruby laser normally emits deep red light near 694 nanometres. Chromium ions within the ruby crystal provide the active laser transition. Its laser emission is monochromatic, coherent and highly directional, unlike ordinary broad-spectrum light sources.
Q90. In optics, which term refers to the diaphragm opening that limits the propagation of light through a lens?
[SSC MTS, 13 Sep 2023]
(A) Collimator
(B) Aperture
(C) Apostilb
(D) Meniscus
Answer: (B) Aperture
Explanation: The aperture is the effective opening admitting light into an optical system. Its diameter governs illumination and influences depth of field and diffraction blur. In cameras, the f-number is focal length divided by the aperture diameter.
Q89. A concave mirror has radius of curvature 50 cm. At what object distance will the reflected image be at infinity?
[UPSC CDS, Sep 2023]
(A) 25 cm
(B) 50 cm
(C) 100 cm
(D) 12.5 cm
Answer: (A) 25 cm
Explanation: The principal focus of a spherical mirror lies halfway between its pole and centre of curvature. Therefore, R = 50 cm gives focal length magnitude 25 cm. Placing the object at that focus produces parallel reflected rays and an image at infinity.
Q88. In a microscope, which objective lens is usually selected first while using the coarse adjustment knob?
[SSC CHSL, 17 Aug 2023]
(A) Oil immersion objective
(B) Low-power objective
(C) Highest-power objective
(D) No objective
Answer: (B) Low-power objective
Explanation: The low-power objective provides a comparatively wide field of view and adequate working distance, making initial focusing safer and easier. Coarse adjustment moves the microscope stage substantially; high-power focusing should generally use the fine-adjustment knob.
Q87. When entering dim surroundings after bright light, which part of the eye changes the pupil size?
[SSC CGL, 25 Jul 2023]
(A) Retina
(B) Iris
(C) Optic nerve
(D) Cornea
Answer: (B) Iris
Explanation: The iris contains muscles that regulate pupil diameter. In bright light it constricts the pupil, whereas in dim surroundings the pupil gradually dilates to admit more light. Rod adaptation also contributes to improved dark sensitivity.
Q86. What is generally formed behind an opaque object when it blocks light?
[SSC MTS, 15 May 2023]
(A) A rainbow
(B) A shadow
(C) A real image in air
(D) A spectrum
Answer: (B) A shadow
Explanation: An opaque object prevents light rays from travelling directly through it, leaving a region of reduced illumination behind. The resulting shadow demonstrates the approximately straight-line propagation of light, with penumbra and umbra depending on source size.
Q85. The SI unit of power of a lens is ______.
[SSC MTS, 10 May 2023]
(A) Candela
(B) Mole
(C) Watt
(D) Dioptre
Answer: (D) Dioptre
Explanation: The dioptre measures reciprocal focal length, with one dioptre equal to one inverse metre. A lens of focal length 0.5 metre has power 2 D. Candela, mole and watt correspond to different physical quantities.
Q84. An image that cannot be captured directly on a screen is called a ______.
[SSC MTS, 9 May 2023]
(A) Virtual image
(B) Real image
(C) Vector image
(D) Spectral image
Answer: (A) Virtual image
Explanation: A virtual image appears where backward extensions of rays meet, even though the actual rays do not intersect there. Because no rays physically converge at its location, a screen cannot directly capture it. Plane mirrors commonly produce virtual images.
Q83. Why is blue light deviated more strongly than red light when passing through a glass prism?
[UPSC NDA, Apr 2023]
(A) Blue has a longer wavelength
(B) Blue travels faster through glass
(C) Their refractive indices are identical
(D) Glass has a larger refractive index for blue than red
Answer: (D) Glass has a larger refractive index for blue than red
Explanation: Ordinary glass exhibits normal dispersion: its refractive index is greater for shorter visible wavelengths. Hence blue light travels more slowly than red within the glass and bends more at prism surfaces, yielding a greater angular deviation.
Q82. Which statements are correct about lens power:
(i) a convex lens has positive power;
(ii) a concave lens has negative power?
[SSC CHSL, 17 Mar 2023]
(A) Both (i) and (ii)
(B) Only (i)
(C) Only (ii)
(D) Neither
Answer: (A) Both (i) and (ii)
Explanation: Lens power P = 1/f carries the same sign as focal length. A convex lens in air converges parallel rays and has positive focal length, whereas a concave lens diverges them and has negative focal length. Both statements are correct.
Q81. A convex mirror always forms an image that is ______.
[SSC CHSL, 16 Mar 2023]
(A) Real and inverted
(B) Real and magnified
(C) Virtual and erect
(D) Virtual and inverted
Answer: (C) Virtual and erect
Explanation: The convex surface reflects rays away from the axis, producing diverging reflected rays. Their extensions meet behind the mirror and create an erect virtual image, always smaller than a real object. This makes convex mirrors suitable for vehicle rear-view applications.
Q80. Which defect of vision is commonly corrected using a bifocal lens?
[SSC CGL, 2023]
(A) Myopia only
(B) Astigmatism only
(C) Presbyopia
(D) Colour blindness
Answer: (C) Presbyopia
Explanation: Presbyopia results from age-related loss of flexibility of the eye lens and reduced accommodation. Bifocal spectacles can provide separate optical powers for near and far viewing, allowing a person to see objects at both distances more clearly.
Q79. Which mirror always produces an erect, diminished, virtual image of a real object?
[SSC CGL, 2023]
(A) Concave mirror
(B) Convex mirror
(C) Plane mirror
(D) Parabolic concave mirror
Answer: (B) Convex mirror
Explanation: A convex mirror has an outward-bulging reflecting surface. The reflected rays diverge and produce a virtual upright image located behind the mirror between pole and focus. Because the image is smaller, more surroundings fit into its field of view.
Q78. Which property of light remains unchanged when it refracts from one transparent medium to another?
[WBCS Mains Science, 2023]
(A) Speed
(B) Frequency
(C) Wavelength
(D) Direction for oblique incidence
Answer: (B) Frequency
Explanation: The frequency of a wave is set by its source and must remain continuous across a stationary boundary. When light enters another medium, its speed and wavelength change together, preserving f = v/λ. Refraction may also change direction.
Q77. Which of the following statements concerning polarisation of light and sound is correct?
[WBCS Mains Science, 2023]
(A) Both light and sound waves in air can be polarised
(B) Only sound in air can be polarised
(C) Light can be polarised; ordinary sound in air cannot
(D) Neither can be polarised
Answer: (C) Light can be polarised; ordinary sound in air cannot
Explanation: Light is a transverse electromagnetic wave, so its electric-field oscillations can be confined to one plane. Ordinary sound in air consists of longitudinal pressure waves and lacks the transverse oscillation needed for conventional polarisation.
Q76. Why do stars appear to twinkle?
[UPSC CDS, 10 Apr 2022]
(A) Scattering by the Moon
(B) Atmospheric refraction
(C) Reflection by water droplets only
(D) Dispersion in a glass prism
Answer: (B) Atmospheric refraction
Explanation: Starlight crosses atmospheric layers whose temperature, density and refractive index change irregularly. The resulting changes in ray direction and intensity make a point-like star seem to fluctuate in position and brightness. Planets generally twinkle less because they subtend larger angles.
Q75. Which inequality approximately represents the near-field Fresnel diffraction region for gap g, slit width W and wavelength λ?
[UPPSC Polytechnic Lecturer, 22 Mar 2022]
(A) g exceeds W²/λ greatly
(B) g is much smaller than λ
(C) λ is much smaller than g, which is smaller than W²/λ
(D) g always equals λ
Answer: (C) λ is much smaller than g, which is smaller than W²/λ
Explanation: Fresnel diffraction describes near-field patterns for which wavefront curvature matters. In a commonly used proximity-imaging geometry, a characteristic range is λ much less than g and g less than W²/λ. Fraunhofer diffraction describes the far-field limit.
Q74. The phenomenon in which a prism separates white light into different constituent colours is called ______.
[RRB Group D, 2022]
(A) Reflection
(B) Polarisation
(C) Refraction alone
(D) Dispersion
Answer: (D) Dispersion
Explanation: Dispersion occurs because a transparent material’s refractive index depends on wavelength. Thus different colours bend through different angles at prism surfaces. Their separation into an ordered visible spectrum is distinct from simple reflection or absorption.
Q73. Which of the following is NOT primarily a result of scattering of light?
[RRB Group D, 2022]
(A) Blue colour of clear sky
(B) Formation of a rainbow
(C) Reddish hue of the setting Sun
(D) Tyndall effect in a colloid
Answer: (B) Formation of a rainbow
Explanation: A rainbow mainly arises from refraction, dispersion and internal reflection within raindrops. Blue sky, reddish sunsets and the visible path of a beam in a colloid all involve scattering of light, though in different particle-size conditions.
Q72. A prism forms a spectrum because the refractive index for violet light is ________ that for red light.
[RRB Group D, 2022]
(A) Lower than
(B) Equal to
(C) Greater than
(D) Always zero
Answer: (C) Greater than
Explanation: Glass typically has a higher refractive index for violet than for red light. Consequently, violet rays bend more strongly at the prism’s sloping surfaces, separating from red and producing a visible spectrum instead of a single white ray.
Q71. A light ray moves from glass to air and bends away from the normal. What can be said about the relative refractive index of air with respect to glass?
[RRB Group D, 2022]
(A) It is greater than two
(B) It equals one
(C) It is negative
(D) It is less than one
Answer: (D) It is less than one
Explanation: Air is optically rarer than glass, with a smaller refractive index. Entering air, light speeds up and refracts away from the normal. Therefore the ratio n(air)/n(glass) is less than one, provided transmission rather than total reflection occurs.
Q70. A light ray strikes glass from air at 60 degrees to the normal. If glass has refractive index √3, what is the refracted angle?
[RRB Group D, 2022]
(A) 60°
(B) 45°
(C) 30°
(D) 90°
Answer: (C) 30°
Explanation: Snell’s law gives sin r = sin 60°/√3 = (√3/2)/√3 = 1/2. Thus r = 30°. Light bends toward the normal because glass has greater refractive index than air.
Q69. A light ray travelling through an equilateral prism parallel to the base has what relationship between incidence and emergence angles?
[RRB Group D, 2022]
(A) Incidence exceeds emergence
(B) Emergence exceeds incidence
(C) The angles are equal
(D) Both must be 90°
Answer: (C) The angles are equal
Explanation: For a symmetrical ray path through an equilateral prism, the incident and emergent angles are equal. The internal refracted ray is parallel to the base, corresponding to the condition of minimum deviation for the prism.
Q68. The Sun appears flattened near the horizon because of which optical effect?
[RRB Group D, 2022]
(A) Diffraction
(B) Atmospheric refraction
(C) Total internal reflection
(D) Polarisation
Answer: (B) Atmospheric refraction
Explanation: The lower edge of the solar disc lies closer to the horizon and is refracted more than the upper edge. Unequal vertical displacement compresses the disc’s apparent height, making the setting or rising Sun look flattened.
Q67. Which optical phenomenon explains why sunrise is observed earlier than the geometric sunrise?
[RRB Group D, 2022]
(A) Diffraction
(B) Dispersion
(C) Scattering
(D) Atmospheric refraction
Answer: (D) Atmospheric refraction
Explanation: Atmospheric refractive index increases toward denser, lower layers of air, bending sunlight along a curved path. An observer sees the Sun at an apparent position above its actual location, allowing visibility slightly before the geometric sunrise.
Q66. When light passes through a triangular glass prism, how many principal refractions occur?
[RRB Group D, 2022]
(A) One
(B) Two
(C) Three
(D) Four
Answer: (B) Two
Explanation: A ray usually refracts on entering the glass from air and refracts again upon emerging into air. The two nonparallel prism faces cause a net change in direction. Dispersion accompanies refraction when the source contains multiple wavelengths.
Q65. What is the name of the angle between original incident direction and emergent direction from a prism?
[RRB Group D, 2022]
(A) Critical angle
(B) Angle of incidence
(C) Angle of reflection
(D) Angle of deviation
Answer: (D) Angle of deviation
Explanation: The angle of deviation expresses the total change in a ray’s direction as it passes through a prism. It is measured between the extended original incident ray and the emergent ray, not between a ray and a surface normal.
Q64. Light travels in a transparent medium of refractive index 1.33. Approximately what is its speed, if c = 3 × 10⁸ m/s?
[RRB Group D, 2022]
(A) 2.26 × 10⁸ m/s
(B) 3.99 × 10⁸ m/s
(C) 1.33 × 10⁸ m/s
(D) 4.33 × 10⁸ m/s
Answer: (A) 2.26 × 10⁸ m/s
Explanation: By definition n = c/v, so v = c/n. Dividing 3 × 10⁸ by 1.33 gives approximately 2.26 × 10⁸ metres per second. Light moves more slowly in the medium than in vacuum.
Q63. Why do objects viewed through heated air above a fire seem to flicker or dance?
[RRB Group D, 2022]
(A) Polarisation
(B) Atmospheric refraction
(C) Total internal reflection in glass
(D) Absorption only
Answer: (B) Atmospheric refraction
Explanation: Hot and cool air layers have differing and rapidly varying refractive indices. As light crosses these turbulent layers it bends irregularly, shifting the apparent positions of objects from moment to moment and producing a shimmering appearance.
Q62. When a ray enters a prism, which effect becomes more pronounced if the prism material’s refractive index increases?
[RRB Group D, 2022]
(A) No deviation
(B) Lesser deviation
(C) Greater deviation
(D) Complete disappearance of the ray
Answer: (C) Greater deviation
Explanation: A material with a larger refractive index typically bends incident light more strongly at a given angle, subject to the prism geometry. The resulting angular deviation increases under comparable conditions, because light slows more markedly relative to air.
Q61. Which phenomenon makes the sky blue and the setting Sun reddish, and also helps explain the Tyndall effect?
[RRB Group D, 2022]
(A) Scattering of light
(B) Diffraction only
(C) Polarisation only
(D) Nuclear radiation
Answer: (A) Scattering of light
Explanation: Different forms of scattering redistribute incoming light. Tiny atmospheric molecules scatter blue wavelengths more strongly, while light reaching observers at sunset has lost much of its blue component. Larger colloidal particles make a passing light beam visible.
Q60. For scattering by very small particles, which visible wavelengths are scattered more strongly?
[RRB Group D, 2022]
(A) Red and orange
(B) Violet and blue
(C) All equally regardless of wavelength
(D) Infrared only
Answer: (B) Violet and blue
Explanation: Rayleigh scattering from particles much smaller than light’s wavelength scales approximately with 1/λ⁴. Consequently blue and violet are scattered more strongly than red. Eye sensitivity and absorption help explain why the daylight sky usually looks blue.
Q59. Which optical devices can have a positive focal length under the Cartesian convention?
[RRB Group D, 2022]
(A) Concave lens and concave mirror
(B) Concave lens and plane mirror
(C) Concave mirror and plane mirror
(D) Convex lens and convex mirror
Answer: (D) Convex lens and convex mirror
Explanation: A convex lens has a real focus on the outgoing-light side and therefore a positive focal length. A convex mirror has a virtual focus behind its surface, also positive for mirrors under the usual Cartesian convention.
Q58. A ray passing through the optical centre of a thin lens emerges ________ the original direction.
[RRB Group D, 2022]
(A) At right angles to
(B) Parallel but shifted far away
(C) With its colour changed
(D) Along the same straight line
Answer: (D) Along the same straight line
Explanation: The optical centre is the point at which a paraxial ray passes through a thin lens without appreciable net angular deviation. It is one of the main construction rays used to find image positions in lens diagrams.
Q57. What is the straight line joining the two principal focal points of a thin lens called?
[RRB Group D, 2022]
(A) Principal axis
(B) Optical power
(C) Angle of emergence
(D) Radius of curvature
Answer: (A) Principal axis
Explanation: The principal axis is the reference line passing through the optical centre and the centres of curvature of the lens surfaces. Its two focal points lie on this axis, providing the geometry for standard ray-diagram constructions.
Q56. A spherical mirror forms an image with magnification −1.38. Which description fits the image?
[RRB Group D, 2022]
(A) Virtual, erect and smaller
(B) Virtual, erect and larger
(C) Real, inverted and larger
(D) Real, inverted and smaller
Answer: (C) Real, inverted and larger
Explanation: Mirror magnification m = −v/u includes orientation through its sign. A negative value means the image is inverted and hence real for a single spherical mirror with a real object. Since its magnitude exceeds one, the image is enlarged.
Q55. A lens has power −6.5 dioptres. What is its approximate focal length?
[RRB Group D, 2022]
(A) +15.38 cm
(B) −15.38 cm
(C) −65 cm
(D) +65 cm
Answer: (B) −15.38 cm
Explanation: The focal length is f = 1/P = −1/6.5 metre, approximately −0.1538 metre or −15.38 centimetres. The negative sign shows that the lens is diverging, as expected for a concave lens in air.
Q54. Which mirror produces a virtual and highly diminished image of an object at infinity?
[RRB Group D, 2022]
(A) Plane mirror
(B) Concave mirror
(C) Convex mirror
(D) Parabolic concave mirror
Answer: (C) Convex mirror
Explanation: Parallel rays from a distant object diverge after reflecting from a convex mirror, appearing to originate from its virtual focus. Because the object’s angular extent is tiny, the resulting image is highly diminished and cannot be captured on a screen.
Q53. What is the focal length of a convex lens having power +3 D, approximately?
[RRB Group D, 2022]
(A) 3 m
(B) 0.03 m
(C) −0.33 m
(D) +0.33 m
Answer: (D) +0.33 m
Explanation: Power and focal length are reciprocals when the latter is in metres. Therefore f = 1/3 metre, approximately +0.33 metre. A positive value identifies a converging convex lens, which can form real images for appropriate object distances.
Q52. Where must an object be placed before a concave mirror to obtain a real, inverted image of the same size?
[RRB Group D, 2022]
(A) Between F and C
(B) Between pole and focus
(C) At infinity
(D) At centre of curvature
Answer: (D) At centre of curvature
Explanation: For a concave mirror with the object at C, reflected rays meet at C as well. The real image is inverted and equal in height to the object, since the magnification is −1. This is a standard spherical-mirror image case.
Q51. What sign does the focal length of a convex mirror have in the Cartesian sign convention?
[RRB Group D, 2022]
(A) Positive
(B) Negative
(C) Always zero
(D) Imaginary only
Answer: (A) Positive
Explanation: The convex mirror’s focus is virtual and situated behind its reflecting surface. This position lies along the positive direction of the principal axis under the common mirror convention. Therefore the mirror’s focal length is positive.
Q50. Which two kinds of mirrors always form a virtual, upright image of a real object?
[RRB Group D, 2022]
(A) Concave and convex
(B) Plane and concave
(C) Plane and convex
(D) Concave and parabolic
Answer: (C) Plane and convex
Explanation: A plane mirror produces a virtual upright image of unchanged size, while a convex mirror produces a virtual upright reduced image. A concave mirror can instead generate real, inverted images when its object lies beyond the focal point.
Q49. What is the SI unit of focal length?
[RRB Group D, 2022]
(A) Metre
(B) Dioptre
(C) Candela
(D) Radian
Answer: (A) Metre
Explanation: Focal length is a distance measured between an optical centre or mirror pole and its principal focus. Therefore its SI unit is the metre. Dioptre, in contrast, expresses inverse metres and measures optical power.
Q48. What is the focal length of a concave lens whose power is −5 D?
[RRB Group D, 2022]
(A) +20 cm
(B) −5 cm
(C) +5 cm
(D) −20 cm
Answer: (D) −20 cm
Explanation: The thin-lens formula gives f = 1/P. For −5 D, f = −0.2 metre = −20 centimetres. Negative focal length is characteristic of a diverging lens, with its virtual focus on the incident-light side.
Q47. An object placed very far from a concave mirror forms an image where?
[RRB Group D, 2022]
(A) At centre of curvature
(B) Behind the mirror
(C) At the principal focus
(D) At twice its focal length
Answer: (C) At the principal focus
Explanation: Incident rays from a very distant object are nearly parallel to a concave mirror’s principal axis. Reflection makes them converge at the principal focus, yielding a real, inverted and extremely small image near F.
Q46. Splitting white light into component colours on passing through a prism is due to ________.
[WBCS Mains 2021, held May 2022]
(A) Reflection
(B) Refraction only
(C) Transmission
(D) Dispersion
Answer: (D) Dispersion
Explanation: Dispersion means wavelength-dependent deviation, because the refractive index of glass changes with colour. Violet typically bends more than red, allowing a prism to resolve a single beam of white light into a coloured spectrum.
Q45. Who among the following made the first compound microscope?
[SSC MTS, 2021]
(A) William Gilbert
(B) Zacharias Janssen
(C) John Harington
(D) Galileo Galilei
Answer: (B) Zacharias Janssen
Explanation: Zacharias Janssen, often working in accounts with his father Hans, is traditionally associated with early compound microscopes. Such instruments use an objective and eyepiece to magnify small objects in successive stages, unlike a single-lens magnifier.
Q44. People who wear bifocal lenses commonly have which age-related eye condition?
[SSC MTS, 14 Oct 2021]
(A) Hypermetropia only
(B) Myopia only
(C) Presbyopia
(D) Cataract
Answer: (C) Presbyopia
Explanation: Presbyopia involves reduced near focusing caused by age-related stiffening of the crystalline lens. Bifocal spectacles combine portions of different optical powers, typically aiding distant and close vision without changing glasses each time.
Q43. What is the coloured ring behind the cornea that changes the size of the pupil?
[SSC MTS, 8 Oct 2021]
(A) Retina
(B) Iris
(C) Fovea
(D) Choroid
Answer: (B) Iris
Explanation: The iris is the coloured muscular diaphragm of the eye. It contracts or relaxes to vary pupil size and thereby controls the amount of light entering. The retina is the light-sensitive layer where images are formed.
Q42. The spectrum of colours on a compact disc (CD) is mainly due to ______.
[WBCS Prelims, 22 Aug 2021]
(A) Refraction
(B) Reflection
(C) Diffraction
(D) Polarisation
Answer: (C) Diffraction
Explanation: Closely spaced tracks on a compact disc behave as a diffraction grating. Light reflected from the tracks interferes, with each wavelength reinforced in different directions. Thus shifting the disc or the viewing angle reveals bands of different colours.
Q41. Which electromagnetic radiation is used in most television remote controls?
[RRB NTPC, 31 Jul 2021]
(A) X-rays
(B) Ultraviolet
(C) Gamma rays
(D) Infrared
Answer: (D) Infrared
Explanation: Common television remotes encode signals as pulses of infrared light from an LED. An infrared receiver in the television detects those pulses and decodes commands. Infrared wavelengths are longer than visible red light and are invisible to human eyes.
Q40. Why does the clear daytime sky look blue?
[RRB NTPC, 7 Apr 2021]
(A) Scattering of sunlight
(B) Total reflection at the ground
(C) Interference of clouds only
(D) Dispersion by a glass lens
Answer: (A) Scattering of sunlight
Explanation: Air molecules scatter the shorter visible wavelengths of sunlight more strongly than longer red wavelengths. Scattered blue light reaches the observer from many directions, giving the sky its characteristic colour. This mechanism is called Rayleigh scattering.
Q39. Which equation gives the thin-lens formula under Cartesian sign convention?
[RRB NTPC, 27 Mar 2021]
(A) 1/v + 1/f = 1/u
(B) 1/v − 1/u = 1/f
(C) 1/h + 1/v = 1/f
(D) 1/v + 1/u = 1/f
Answer: (B) 1/v − 1/u = 1/f
Explanation: The thin-lens formula is 1/v − 1/u = 1/f, connecting image distance v, object distance u and focal length f. Proper signs allow use with convex or concave lenses and with real or virtual images.
Q38. Light travels in a ______ through a homogeneous medium.
[RRB NTPC, 12 Mar 2021, Shift 1]
(A) Curved line
(B) Horizontal line only
(C) Straight line
(D) Vertical line only
Answer: (C) Straight line
Explanation: In a uniform transparent medium, a light ray travels approximately along a straight path, described as rectilinear propagation. It explains ordinary sharp shadows and pinhole-camera images. Boundaries, refractive-index gradients and diffraction can alter the path.
Q37. Which lens is suitable for correcting myopia?
[RRB NTPC, 12 Mar 2021, Shift 2]
(A) Compound lens
(B) Convex lens
(C) Goggles without corrective power
(D) Concave lens
Answer: (D) Concave lens
Explanation: Myopia means distant objects focus in front of the retina, often because the eye is too long or too strongly converging. A concave lens spreads incoming light slightly, shifting the final focus back onto the retina.
Q36. What kind of mirror is placed behind the light source in a car headlight?
[RRB NTPC, 2021]
(A) Spherical convex mirror
(B) Cylindrical mirror
(C) Parabolic concave mirror
(D) Plane mirror
Answer: (C) Parabolic concave mirror
Explanation: A parabolic concave reflector converts light from a source near its focus into an approximately parallel beam. This directs more illumination forward along the road, unlike a convex mirror, which spreads reflected light over a wider angle.
Q35. Which of the following phenomena is NOT caused by refraction?
[RRB NTPC, 19 Jan 2021]
(A) A pencil appearing bent in water
(B) Apparent depth of a pool
(C) Twinkling of stars
(D) Red colour of the setting Sun
Answer: (D) Red colour of the setting Sun
Explanation: A red sunset mainly comes from scattering: blue wavelengths are removed from the direct beam over a long atmospheric path. The other effects arise from rays changing direction at boundaries or through layers of varying atmospheric refractive index.
Q34. What is another name for lazy eye?
[SSC CHSL, Oct 2020]
(A) Myopia
(B) Hypermetropia
(C) Astigmatism
(D) Amblyopia
Answer: (D) Amblyopia
Explanation: Amblyopia is a developmental reduction of vision in one eye, sometimes both, when visual processing fails to develop normally during childhood. It is not simply a refractive error, although uncorrected unequal refractive errors can contribute to it.
Q33. A glass lens disappears visually when immersed in a liquid of what refractive index relative to the glass?
[RPSC 2nd Grade Science, 4 Jul 2019]
(A) Equal to that of glass
(B) Much lower
(C) Slightly different
(D) Always zero
Answer: (A) Equal to that of glass
Explanation: When a transparent object and surrounding liquid have identical refractive indices, rays passing through their interface suffer no deviation. The lens becomes difficult to distinguish because its boundaries lose optical contrast, though absorption or imperfections may remain visible.
Q32. Which light-sensitive pigment is found in the rod cells of the human retina?
[SSC CHSL, 3 Jul 2019]
(A) Haemoglobin
(B) Melanin
(C) Chlorophyll
(D) Rhodopsin
Answer: (D) Rhodopsin
Explanation: Rhodopsin is a light-sensitive pigment in retinal rod cells. It is particularly important for vision in dim illumination. Absorbing photons initiates a photochemical signalling cascade; cone cells instead support detailed colour vision in brighter light.
Q31. What physical quantity is measured by a light-year?
[WBCS Prelims, 2019]
(A) Light intensity
(B) Time
(C) Distance
(D) Frequency
Answer: (C) Distance
Explanation: A light-year is the distance light travels in vacuum in one Julian year, roughly 9.46 trillion kilometres. Despite including the word year, it is a length unit used in astronomy to describe large distances between celestial objects.
Q30. The focal length of a concave mirror is f in air. What is its focal length in water?
[RPSC 2nd Grade Science, 1 Nov 2018]
(A) f
(B) 3f/4
(C) 4f/3
(D) 7f/3
Answer: (A) f
Explanation: Reflection depends on the geometry of the mirror and the law of reflection, so changing the surrounding transparent medium does not change a spherical mirror’s focal length. Lenses differ because refraction depends on relative refractive indices.
Q29. Which type of spherical mirror converges light rays parallel to its principal axis?
[RRB Group D, 2018]
(A) Convex mirror
(B) Concave mirror
(C) Plane mirror
(D) Transparent glass slab
Answer: (B) Concave mirror
Explanation: A concave mirror reflects parallel rays inward so they meet at a real principal focus. This focusing action makes it a converging mirror, useful in solar concentrators and optical reflectors. A convex mirror instead causes reflected rays to diverge.
Q28. Which is NOT a usual application of a concave mirror?
[RRB Group D, 2018]
(A) Dentist’s examining mirror
(B) Shaving mirror
(C) Vehicle rear-view safety mirror
(D) Reflector in searchlights
Answer: (C) Vehicle rear-view safety mirror
Explanation: An ordinary vehicle rear-view safety mirror is convex because it provides a wide field of view. Concave mirrors may magnify nearby objects or concentrate reflected light, properties useful in examination mirrors and searchlight reflectors.
Q27. Which spherical mirror is known as a diverging mirror?
[RRB Group D, 2018]
(A) Concave
(B) Convex
(C) Plane
(D) Parabolic concave
Answer: (B) Convex
Explanation: A convex mirror reflects incident rays so they spread farther apart. Their backward extensions intersect behind the mirror at a virtual focus. This diverging behaviour explains why images are upright and diminished over a large field of view.
Q26. The image formed by a plane mirror is generally ________.
[RRB Group D, 2018]
(A) Virtual and laterally inverted
(B) Real and inverted vertically
(C) Real and magnified
(D) Virtual and upside down
Answer: (A) Virtual and laterally inverted
Explanation: A plane mirror forms a virtual upright image with equal height and apparent left-right reversal, called lateral inversion. The image seems to lie behind the mirror at the same distance as the object stands in front.
Q25. Which equation is the spherical-mirror formula?
[RRB Group D, 2018]
(A) 1/f = 1/u − 1/v
(B) 1/f = v − u
(C) 1/f = 1/u + 1/v
(D) f = u + v
Answer: (C) 1/f = 1/u + 1/v
Explanation: The mirror equation is 1/f = 1/u + 1/v when Cartesian signs are applied. It relates the mirror’s focal length to object and image distances and works for convex and concave spherical mirrors under paraxial conditions.
Q24. Why does a light ray bend toward the normal when it enters an optically denser material?
[RRB Group D, 2018]
(A) Because of reflection
(B) Because of polarisation
(C) Because of refraction
(D) Because its frequency rises
Answer: (C) Because of refraction
Explanation: On crossing from an optically rarer medium into a denser one, light slows and usually bends toward the surface normal. This change in direction is refraction. Its frequency remains unchanged across the boundary, while its wavelength becomes shorter.
Q23. Which material is generally unsuitable for manufacturing transparent optical lenses?
[RRB Group D, 2018]
(A) Glass
(B) Soil
(C) Transparent plastic
(D) Quartz
Answer: (B) Soil
Explanation: Useful ordinary lenses must transmit visible light and possess a suitable refractive index. Soil is opaque and heterogeneous, scattering or absorbing light rather than forming a clear refracted image. Glass, quartz and suitable plastics can be shaped into lenses.
Q22. Why can the Sun be seen about two minutes before sunrise and after sunset?
[RRB Group D, 2018]
(A) Atmospheric refraction
(B) Diffraction at the horizon
(C) Reflection by the Moon
(D) Dispersion into a rainbow
Answer: (A) Atmospheric refraction
Explanation: Atmospheric layers bend sunlight rays, making the Sun appear higher than its geometrical position. This shifts apparent sunrise earlier and apparent sunset later by about two minutes each under standard conditions, although atmospheric conditions alter the exact effect.
Q21. What is the chief cause of the twinkling of stars?
[RRB Group D, 2018]
(A) Interference at the eye
(B) Scattering inside stars
(C) Reflection by clouds
(D) Atmospheric refraction
Answer: (D) Atmospheric refraction
Explanation: Starlight encounters changing atmospheric density and temperature layers, which repeatedly alter its path. The star is nearly a point source, so these variations appear as rapid fluctuations in brightness and apparent position known as scintillation.
Q20. Which lens is used as a simple magnifying glass?
[RRB Group D, 2018]
(A) Concave lens with long focal length
(B) Plane glass sheet
(C) Cylindrical mirror
(D) Convex lens with short focal length
Answer: (D) Convex lens with short focal length
Explanation: A short-focal-length convex lens forms a magnified upright virtual image when an object is placed within its focal length. This makes fine details easier to see. Its magnifying power increases as the focal length becomes shorter.
Q19. Which optical phenomenon mainly explains the white appearance of most clouds?
[RRB Group D, 2018]
(A) Refraction only
(B) Scattering of many wavelengths
(C) Nuclear emission
(D) Total internal reflection only
Answer: (B) Scattering of many wavelengths
Explanation: Cloud droplets are comparatively large compared with visible wavelengths and scatter a broad range of colours similarly. Since the scattered components remain mixed, the combined light appears white or grey, depending on illumination and cloud thickness.
Q18. A concave spherical mirror has radius of curvature 6.2 cm. What is its focal length in magnitude?
[RRB Group D, 2018]
(A) 3.1 cm
(B) 6.2 cm
(C) 12.4 cm
(D) 0.31 cm
Answer: (A) 3.1 cm
Explanation: For a spherical mirror, the focal length is half the radius of curvature. Therefore focal-length magnitude = 6.2/2 = 3.1 cm. A concave mirror has negative signed focal length using the Cartesian convention, but the magnitude is positive.
Q17. For a convex lens, if x1 and x2 are distances from the first and second foci to conjugate object and image points, which condition holds?
[UPPSC LT Grade Science, 29 Jul 2018]
(A) x1f1 = x2f2
(B) x1f2 = x2f1
(C) x2 − x1 = f2 − f1
(D) x1x2 = f1f2
Answer: (D) x1x2 = f1f2
Explanation: Newton’s form of the lens equation expresses conjugate distances measured from the focal points. Their product equals the product of the first and second focal lengths. In the same medium on both sides, this simplifies to x1x2 = f².
Q16. A simple periscope works principally on which phenomenon?
[UPSC NDA, 15 Apr 2018]
(A) Reflection
(B) Refraction
(C) Dispersion
(D) Polarisation
Answer: (A) Reflection
Explanation: A simple periscope uses two plane mirrors or prisms arranged to redirect incoming light twice. Because reflected rays preserve angle relationships, the observer can see over an obstruction without a direct line of sight. The key process is reflection.
Q15. The refractive index of an ordinary transparent medium with respect to air is normally ______.
[UPSC NDA, 15 Apr 2018]
(A) Less than zero
(B) Zero
(C) Exactly one
(D) Greater than one
Answer: (D) Greater than one
Explanation: Light ordinarily travels slower in glass or water than in air. Their refractive index relative to air is therefore the speed in air divided by the lower speed in the material, giving a ratio greater than unity.
Q14. A ray travelling from an optically denser to a rarer medium normally ______.
[CGPSC Civil Services, 2018]
(A) Speeds up and bends away from the normal
(B) Speeds up and bends toward the normal
(C) Slows down and bends away from the normal
(D) Slows down and bends toward the normal
Answer: (A) Speeds up and bends away from the normal
Explanation: In an optically rarer medium, light’s speed increases. Snell’s law then gives a larger refraction angle measured from the normal, so the ray bends away from it. At sufficiently large incidence angles, total internal reflection may instead occur.
Q13. Which claim about ultraviolet light is incorrect?
[UPSC CDS, Nov 2017]
(A) It has higher frequency than visible red
(B) It has shorter wavelength than visible light
(C) It may cause fluorescence
(D) Its photons have less energy than visible-light photons
Answer: (D) Its photons have less energy than visible-light photons
Explanation: Photon energy obeys E = hf. Ultraviolet radiation has a higher frequency and shorter wavelength than visible light, so an ultraviolet photon carries more, not less, energy. That higher energy makes UV effective in certain photochemical processes.
Q12. A plane mirror is turned through an angle. The angle of incidence is 40°. What angle separates incident and reflected rays measured by the stated deviation?
[UPSC CDS, Nov 2017]
(A) 80°
(B) 100°
(C) 40°
(D) 140°
Answer: (B) 100°
Explanation: The angle between an incident ray and its reflected ray is 2i, or 80° for incidence of 40°. The deviation from a ray’s original straight-line direction is 180° − 2i, which is 100°. The question asks for deviation.
Q11. Which kind of mirror is used on the outside of vehicles to obtain a wider view?
[SSC CGL, Aug 2017]
(A) Convex mirror
(B) Concave mirror
(C) Plane mirror only
(D) Parabolic concave mirror
Answer: (A) Convex mirror
Explanation: Convex mirrors form diminished images and accept rays from a broad range of directions. Consequently, a small side-view mirror reveals more traffic behind the vehicle. The images remain upright but may make objects seem more distant.
Q10. Which of the following are the two main types of spherical mirrors?
[SSC CGL, Aug 2017]
(A) Plane and curved
(B) Concave and convex
(C) Converging lenses and diverging lenses
(D) Glass and metal
Answer: (B) Concave and convex
Explanation: Spherical mirrors are classified by whether the reflecting surface curves inward or outward. A concave mirror generally converges paraxial rays and a convex mirror diverges them. Both are portions of an underlying spherical surface.
Q9. Which mirror can produce a magnified inverted image of a real object?
[SSC CGL, Aug 2017]
(A) Concave mirror
(B) Convex mirror
(C) Plane mirror
(D) Any mirror at any distance
Answer: (A) Concave mirror
Explanation: A concave mirror forms a real inverted image larger than the object when the object lies between its focus and centre of curvature. Convex and plane mirrors form virtual upright images for real objects and cannot produce this case.
Q8. Which of the wavelengths 200 nm, 500 nm and 1000 nm lies in the visible region?
[UPSC CDS, Feb 2017]
(A) 200 nm only
(B) 1000 nm only
(C) 500 nm only
(D) All three
Answer: (C) 500 nm only
Explanation: The visible spectrum extends approximately from 380 to 750 nanometres. Hence 500 nm lies in the visible region, whereas 200 nm is ultraviolet and 1000 nm is infrared. Visible wavelengths correspond to the colours sensed by the human eye.
Q7. The power of a lens is measured in ______.
[MPPSC State Service, 2017]
(A) Dioptre
(B) Ion
(C) Lumen
(D) Candela
Answer: (A) Dioptre
Explanation: The unit of lens power is the dioptre, symbol D. The value is the reciprocal of focal length in metres, making its dimensional unit m⁻¹. Lumen measures luminous flux and candela measures luminous intensity.
Q6. Myopia is corrected by using which optical device?
[CGPSC Prelims, 2016]
(A) Concave lens
(B) Concave mirror
(C) Convex lens
(D) Convex mirror
Answer: (A) Concave lens
Explanation: In myopia, the eye focuses distant light in front of the retina instead of on it. A concave corrective lens causes incident rays to diverge slightly before entering the eye, allowing the combined system to focus on the retina.
Q5. A mirage is an example of which phenomenon?
[RPSC Assistant Engineer, 2013]
(A) Light reflection only
(B) Light dispersion
(C) Light refraction
(D) Light scattering
Answer: (C) Light refraction
Explanation: A mirage arises when light passes through air layers of differing temperature and refractive index. Rays bend continuously, sometimes with conditions resembling total internal reflection. The observer perceives a displaced or inverted image that can resemble water on hot roads.
Q4. Night-vision instruments often use which part of the electromagnetic spectrum?
[UPSC CSE Prelims, 2009]
(A) Radio waves
(B) Microwaves
(C) Infrared radiation
(D) Gamma rays
Answer: (C) Infrared radiation
Explanation: Infrared radiation carries thermal information emitted by warm objects. Many night-vision and thermal-imaging systems detect infrared wavelengths invisible to the human eye, enabling observation in darkness. Some other night-vision systems instead amplify faint visible or near-infrared light.
Q3. Diffusion of light in the atmosphere takes place due to:
[UPSC CSE Prelims, 2003]
(A) Carbon dioxide
(B) Dust particles
(C) Helium
(D) Water vapours
Answer: (B) Dust particles
Explanation: Dust particles and aerosols scatter light in different directions, causing diffusion or diffuse illumination. The scattering changes the direction of sunlight without requiring it to be emitted anew. Moisture droplets may also contribute, but the original question singles out dust particles.
Q2. Which combination of phenomena is chiefly responsible for the formation of a mirage:
(1) heating near the ground,
(2) reflection,
(3) refraction,
(4) diffraction
[UPSC CSE Prelims, 2002]
(A) 1 and 2
(B) 2, 3 and 4
(C) 1 and 3
(D) 4 only
Answer: (C) 1 and 3
Explanation: On a hot day, ground heating creates an air-density gradient close to the surface. Light bends through layers with different refractive indices, producing displaced or inverted images. The main factors are heating and refraction, not ordinary diffraction.
Q1. Which of the following statements are correct:
(1) longer wavelengths scatter more in air;
(2) light speed in water is 95% of c;
(3) oscillating electric currents generate radio waves;
(4) radar can measure speed using the Doppler effect?
[UPSC CSE Prelims, 2002]
(A) 1 and 2
(B) 1 and 3
(C) 2 and 4
(D) 3 and 4
Answer: (D) 3 and 4
Explanation: Small molecules preferentially scatter shorter wavelengths, so statement 1 is false. Light in water travels much slower than 95% of vacuum speed, so 2 is false. Oscillating charges generate radio waves and Doppler radar detects motion, making 3 and 4 correct.