Q70. A light ray strikes glass from air at 60 degrees to the normal. If glass has refractive index √3, what is the refracted angle?
[RRB Group D, 2022]
(A) 60°
(B) 45°
(C) 30°
(D) 90°
Answer: (C) 30°
Explanation: Snell’s law gives sin r = sin 60°/√3 = (√3/2)/√3 = 1/2. Thus r = 30°. Light bends toward the normal because glass has greater refractive index than air.
Q69. A light ray travelling through an equilateral prism parallel to the base has what relationship between incidence and emergence angles?
[RRB Group D, 2022]
(A) Incidence exceeds emergence
(B) Emergence exceeds incidence
(C) The angles are equal
(D) Both must be 90°
Answer: (C) The angles are equal
Explanation: For a symmetrical ray path through an equilateral prism, the incident and emergent angles are equal. The internal refracted ray is parallel to the base, corresponding to the condition of minimum deviation for the prism.
Q68. The Sun appears flattened near the horizon because of which optical effect?
[RRB Group D, 2022]
(A) Diffraction
(B) Atmospheric refraction
(C) Total internal reflection
(D) Polarisation
Answer: (B) Atmospheric refraction
Explanation: The lower edge of the solar disc lies closer to the horizon and is refracted more than the upper edge. Unequal vertical displacement compresses the disc’s apparent height, making the setting or rising Sun look flattened.
Q67. Which optical phenomenon explains why sunrise is observed earlier than the geometric sunrise?
[RRB Group D, 2022]
(A) Diffraction
(B) Dispersion
(C) Scattering
(D) Atmospheric refraction
Answer: (D) Atmospheric refraction
Explanation: Atmospheric refractive index increases toward denser, lower layers of air, bending sunlight along a curved path. An observer sees the Sun at an apparent position above its actual location, allowing visibility slightly before the geometric sunrise.
Q66. When light passes through a triangular glass prism, how many principal refractions occur?
[RRB Group D, 2022]
(A) One
(B) Two
(C) Three
(D) Four
Answer: (B) Two
Explanation: A ray usually refracts on entering the glass from air and refracts again upon emerging into air. The two nonparallel prism faces cause a net change in direction. Dispersion accompanies refraction when the source contains multiple wavelengths.
Q65. What is the name of the angle between original incident direction and emergent direction from a prism?
[RRB Group D, 2022]
(A) Critical angle
(B) Angle of incidence
(C) Angle of reflection
(D) Angle of deviation
Answer: (D) Angle of deviation
Explanation: The angle of deviation expresses the total change in a ray’s direction as it passes through a prism. It is measured between the extended original incident ray and the emergent ray, not between a ray and a surface normal.
Q64. Light travels in a transparent medium of refractive index 1.33. Approximately what is its speed, if c = 3 × 10⁸ m/s?
[RRB Group D, 2022]
(A) 2.26 × 10⁸ m/s
(B) 3.99 × 10⁸ m/s
(C) 1.33 × 10⁸ m/s
(D) 4.33 × 10⁸ m/s
Answer: (A) 2.26 × 10⁸ m/s
Explanation: By definition n = c/v, so v = c/n. Dividing 3 × 10⁸ by 1.33 gives approximately 2.26 × 10⁸ metres per second. Light moves more slowly in the medium than in vacuum.
Q63. Why do objects viewed through heated air above a fire seem to flicker or dance?
[RRB Group D, 2022]
(A) Polarisation
(B) Atmospheric refraction
(C) Total internal reflection in glass
(D) Absorption only
Answer: (B) Atmospheric refraction
Explanation: Hot and cool air layers have differing and rapidly varying refractive indices. As light crosses these turbulent layers it bends irregularly, shifting the apparent positions of objects from moment to moment and producing a shimmering appearance.
Q62. When a ray enters a prism, which effect becomes more pronounced if the prism material’s refractive index increases?
[RRB Group D, 2022]
(A) No deviation
(B) Lesser deviation
(C) Greater deviation
(D) Complete disappearance of the ray
Answer: (C) Greater deviation
Explanation: A material with a larger refractive index typically bends incident light more strongly at a given angle, subject to the prism geometry. The resulting angular deviation increases under comparable conditions, because light slows more markedly relative to air.
Q61. Which phenomenon makes the sky blue and the setting Sun reddish, and also helps explain the Tyndall effect?
[RRB Group D, 2022]
(A) Scattering of light
(B) Diffraction only
(C) Polarisation only
(D) Nuclear radiation
Answer: (A) Scattering of light
Explanation: Different forms of scattering redistribute incoming light. Tiny atmospheric molecules scatter blue wavelengths more strongly, while light reaching observers at sunset has lost much of its blue component. Larger colloidal particles make a passing light beam visible.
