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.
