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.
