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.
