Q110. Two separated convex lenses are arranged for minimum chromatic and spherical aberrations. What is f1:f2?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) 4:1
(B) 2:3
(C) 2:1
(D) 3:1
Answer: (D) 3:1
Explanation: For an appropriately separated pair, zero chromatic aberration requires d = (f1 + f2)/2, while minimum spherical aberration requires d = f1 − f2. Equating these conditions gives f1 = 3f2 and hence a 3:1 ratio.
Q109. How many components does a normal Zeeman spectral line split into?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) Three
(B) Two
(C) Four
(D) Five
Answer: (A) Three
Explanation: In the normal Zeeman effect, an atomic spectral line splits into three components in an external magnetic field. The components correspond to allowed changes in magnetic quantum number of −1, 0 and +1. This demonstrates the magnetic influence on emission frequencies.
Q108. What is the area of the first Fresnel half-period zone on a spherical wavefront?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) 2πbλ
(B) πbλ/2
(C) πb²λ
(D) 4πb/λ
Answer: (A) 2πbλ
Explanation: Successive Fresnel half-period zones contribute wavelets of approximately alternating phase to a point of observation. In the usual spherical-wave construction, the first zone area is expressed as 2Ï€bλ under the problem’s geometry and definitions.
Q107. A quarter-wave plate combined with a Nicol prism can produce which polarisation state?
[UPPSC LT Grade Science, 7 Dec 2025]
(A) Unpolarised light
(B) Circularly polarised light
(C) Only longitudinal waves
(D) Only incoherent light
Answer: (B) Circularly polarised light
Explanation: A Nicol prism produces linearly polarised light. A quarter-wave plate introduces a 90-degree phase difference between perpendicular field components. When the input makes the appropriate 45-degree angle with its axes, the transmitted light becomes circularly polarised.
Q106. Arrange orange (IV), yellow (III), green (II) and blue (I) by increasing deviation through a glass prism.
[UPPSC LT Grade Hindi, 6 Dec 2025]
(A) I, II, III, IV
(B) II, III, IV, I
(C) IV, III, II, I
(D) III, I, IV, II
Answer: (C) IV, III, II, I
Explanation: Redder colours deviate less than bluer colours in ordinary glass because refractive index increases as visible wavelength decreases. Thus among orange, yellow, green and blue, the sequence of increasing prism deviation is orange, yellow, green, then blue.
Q105. Which structure provides fine adjustment of the focal length in the human eye?
[RRB Group D, 5 Dec 2025]
(A) Cornea alone
(B) Iris
(C) Optic nerve
(D) Crystalline eye lens
Answer: (D) Crystalline eye lens
Explanation: The crystalline eye lens adjusts its curvature through the action of ciliary muscles and suspensory structures. This changes optical power to keep objects at different distances in focus on the retina. The iris instead regulates incoming light.
Q104. Under the Cartesian sign convention, distances for a lens are measured from its ________.
[RRB Group D, 4 Dec 2025]
(A) Principal focus
(B) Centre of curvature
(C) Optical centre
(D) Outer rim
Answer: (C) Optical centre
Explanation: The optical centre is the reference point for measuring object and image distances in lens equations. Distances along incident light are positive, opposite distances negative. Measurements should not begin at either focus, which is a separate optical point.
Q103. A concave mirror produces what image if the object lies between its pole and principal focus?
[RRB Group D, 3 Dec 2025]
(A) Real, inverted and enlarged
(B) Real, inverted and reduced
(C) Virtual, erect and enlarged
(D) Virtual, erect and diminished
Answer: (C) Virtual, erect and enlarged
Explanation: If an object lies closer to a concave mirror than its focal point, reflected rays diverge and their extensions meet behind the mirror. This gives an upright magnified virtual image, the condition used in shaving mirrors.
Q102. Which statement about a spherical lens is incorrect?
[RRB Group D, 2 Dec 2025]
(A) It can have a principal axis
(B) It has an optical centre
(C) Its optical centre always coincides with its centre of curvature
(D) It can refract light
Answer: (C) Its optical centre always coincides with its centre of curvature
Explanation: A lens has an optical centre through which a paraxial ray passes nearly undeviated, while its curved surfaces have separate centres of curvature. These locations do not generally coincide. The distinction matters when drawing accurate lens geometry.
Q101. What happens when light passes obliquely through a parallel-sided glass slab?
[RRB Group D, 28 Nov 2025]
(A) It travels faster in glass and emerges perpendicular
(B) It slows in glass and emerges at an unrelated angle
(C) Its speed remains unchanged and it reflects totally
(D) It slows within glass and emerges parallel to the incident ray
Answer: (D) It slows within glass and emerges parallel to the incident ray
Explanation: Light slows and bends toward the normal upon entering glass. At the second parallel surface, it speeds up and bends away from the normal by an equal complementary amount, emerging parallel to the incident ray but laterally displaced.
