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.
