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.
