Q80. Which defect of vision is commonly corrected using a bifocal lens?
[SSC CGL, 2023]
(A) Myopia only
(B) Astigmatism only
(C) Presbyopia
(D) Colour blindness
Answer: (C) Presbyopia
Explanation: Presbyopia results from age-related loss of flexibility of the eye lens and reduced accommodation. Bifocal spectacles can provide separate optical powers for near and far viewing, allowing a person to see objects at both distances more clearly.
Q79. Which mirror always produces an erect, diminished, virtual image of a real object?
[SSC CGL, 2023]
(A) Concave mirror
(B) Convex mirror
(C) Plane mirror
(D) Parabolic concave mirror
Answer: (B) Convex mirror
Explanation: A convex mirror has an outward-bulging reflecting surface. The reflected rays diverge and produce a virtual upright image located behind the mirror between pole and focus. Because the image is smaller, more surroundings fit into its field of view.
Q78. Which property of light remains unchanged when it refracts from one transparent medium to another?
[WBCS Mains Science, 2023]
(A) Speed
(B) Frequency
(C) Wavelength
(D) Direction for oblique incidence
Answer: (B) Frequency
Explanation: The frequency of a wave is set by its source and must remain continuous across a stationary boundary. When light enters another medium, its speed and wavelength change together, preserving f = v/λ. Refraction may also change direction.
Q77. Which of the following statements concerning polarisation of light and sound is correct?
[WBCS Mains Science, 2023]
(A) Both light and sound waves in air can be polarised
(B) Only sound in air can be polarised
(C) Light can be polarised; ordinary sound in air cannot
(D) Neither can be polarised
Answer: (C) Light can be polarised; ordinary sound in air cannot
Explanation: Light is a transverse electromagnetic wave, so its electric-field oscillations can be confined to one plane. Ordinary sound in air consists of longitudinal pressure waves and lacks the transverse oscillation needed for conventional polarisation.
Q76. Why do stars appear to twinkle?
[UPSC CDS, 10 Apr 2022]
(A) Scattering by the Moon
(B) Atmospheric refraction
(C) Reflection by water droplets only
(D) Dispersion in a glass prism
Answer: (B) Atmospheric refraction
Explanation: Starlight crosses atmospheric layers whose temperature, density and refractive index change irregularly. The resulting changes in ray direction and intensity make a point-like star seem to fluctuate in position and brightness. Planets generally twinkle less because they subtend larger angles.
Q75. Which inequality approximately represents the near-field Fresnel diffraction region for gap g, slit width W and wavelength λ?
[UPPSC Polytechnic Lecturer, 22 Mar 2022]
(A) g exceeds W²/λ greatly
(B) g is much smaller than λ
(C) λ is much smaller than g, which is smaller than W²/λ
(D) g always equals λ
Answer: (C) λ is much smaller than g, which is smaller than W²/λ
Explanation: Fresnel diffraction describes near-field patterns for which wavefront curvature matters. In a commonly used proximity-imaging geometry, a characteristic range is λ much less than g and g less than W²/λ. Fraunhofer diffraction describes the far-field limit.
Q74. The phenomenon in which a prism separates white light into different constituent colours is called ______.
[RRB Group D, 2022]
(A) Reflection
(B) Polarisation
(C) Refraction alone
(D) Dispersion
Answer: (D) Dispersion
Explanation: Dispersion occurs because a transparent material’s refractive index depends on wavelength. Thus different colours bend through different angles at prism surfaces. Their separation into an ordered visible spectrum is distinct from simple reflection or absorption.
Q73. Which of the following is NOT primarily a result of scattering of light?
[RRB Group D, 2022]
(A) Blue colour of clear sky
(B) Formation of a rainbow
(C) Reddish hue of the setting Sun
(D) Tyndall effect in a colloid
Answer: (B) Formation of a rainbow
Explanation: A rainbow mainly arises from refraction, dispersion and internal reflection within raindrops. Blue sky, reddish sunsets and the visible path of a beam in a colloid all involve scattering of light, though in different particle-size conditions.
Q72. A prism forms a spectrum because the refractive index for violet light is ________ that for red light.
[RRB Group D, 2022]
(A) Lower than
(B) Equal to
(C) Greater than
(D) Always zero
Answer: (C) Greater than
Explanation: Glass typically has a higher refractive index for violet than for red light. Consequently, violet rays bend more strongly at the prism’s sloping surfaces, separating from red and producing a visible spectrum instead of a single white ray.
Q71. A light ray moves from glass to air and bends away from the normal. What can be said about the relative refractive index of air with respect to glass?
[RRB Group D, 2022]
(A) It is greater than two
(B) It equals one
(C) It is negative
(D) It is less than one
Answer: (D) It is less than one
Explanation: Air is optically rarer than glass, with a smaller refractive index. Entering air, light speeds up and refracts away from the normal. Therefore the ratio n(air)/n(glass) is less than one, provided transmission rather than total reflection occurs.
