Q10. Which of the following are the two main types of spherical mirrors?
[SSC CGL, Aug 2017]
(A) Plane and curved
(B) Concave and convex
(C) Converging lenses and diverging lenses
(D) Glass and metal
Answer: (B) Concave and convex
Explanation: Spherical mirrors are classified by whether the reflecting surface curves inward or outward. A concave mirror generally converges paraxial rays and a convex mirror diverges them. Both are portions of an underlying spherical surface.
Q9. Which mirror can produce a magnified inverted image of a real object?
[SSC CGL, Aug 2017]
(A) Concave mirror
(B) Convex mirror
(C) Plane mirror
(D) Any mirror at any distance
Answer: (A) Concave mirror
Explanation: A concave mirror forms a real inverted image larger than the object when the object lies between its focus and centre of curvature. Convex and plane mirrors form virtual upright images for real objects and cannot produce this case.
Q8. Which of the wavelengths 200 nm, 500 nm and 1000 nm lies in the visible region?
[UPSC CDS, Feb 2017]
(A) 200 nm only
(B) 1000 nm only
(C) 500 nm only
(D) All three
Answer: (C) 500 nm only
Explanation: The visible spectrum extends approximately from 380 to 750 nanometres. Hence 500 nm lies in the visible region, whereas 200 nm is ultraviolet and 1000 nm is infrared. Visible wavelengths correspond to the colours sensed by the human eye.
Q7. The power of a lens is measured in ______.
[MPPSC State Service, 2017]
(A) Dioptre
(B) Ion
(C) Lumen
(D) Candela
Answer: (A) Dioptre
Explanation: The unit of lens power is the dioptre, symbol D. The value is the reciprocal of focal length in metres, making its dimensional unit m⁻¹. Lumen measures luminous flux and candela measures luminous intensity.
Q6. Myopia is corrected by using which optical device?
[CGPSC Prelims, 2016]
(A) Concave lens
(B) Concave mirror
(C) Convex lens
(D) Convex mirror
Answer: (A) Concave lens
Explanation: In myopia, the eye focuses distant light in front of the retina instead of on it. A concave corrective lens causes incident rays to diverge slightly before entering the eye, allowing the combined system to focus on the retina.
Q5. A mirage is an example of which phenomenon?
[RPSC Assistant Engineer, 2013]
(A) Light reflection only
(B) Light dispersion
(C) Light refraction
(D) Light scattering
Answer: (C) Light refraction
Explanation: A mirage arises when light passes through air layers of differing temperature and refractive index. Rays bend continuously, sometimes with conditions resembling total internal reflection. The observer perceives a displaced or inverted image that can resemble water on hot roads.
Q4. Night-vision instruments often use which part of the electromagnetic spectrum?
[UPSC CSE Prelims, 2009]
(A) Radio waves
(B) Microwaves
(C) Infrared radiation
(D) Gamma rays
Answer: (C) Infrared radiation
Explanation: Infrared radiation carries thermal information emitted by warm objects. Many night-vision and thermal-imaging systems detect infrared wavelengths invisible to the human eye, enabling observation in darkness. Some other night-vision systems instead amplify faint visible or near-infrared light.
Q3. Diffusion of light in the atmosphere takes place due to:
[UPSC CSE Prelims, 2003]
(A) Carbon dioxide
(B) Dust particles
(C) Helium
(D) Water vapours
Answer: (B) Dust particles
Explanation: Dust particles and aerosols scatter light in different directions, causing diffusion or diffuse illumination. The scattering changes the direction of sunlight without requiring it to be emitted anew. Moisture droplets may also contribute, but the original question singles out dust particles.
Q2. Which combination of phenomena is chiefly responsible for the formation of a mirage:
(1) heating near the ground,
(2) reflection,
(3) refraction,
(4) diffraction
[UPSC CSE Prelims, 2002]
(A) 1 and 2
(B) 2, 3 and 4
(C) 1 and 3
(D) 4 only
Answer: (C) 1 and 3
Explanation: On a hot day, ground heating creates an air-density gradient close to the surface. Light bends through layers with different refractive indices, producing displaced or inverted images. The main factors are heating and refraction, not ordinary diffraction.
Q1. Which of the following statements are correct:
(1) longer wavelengths scatter more in air;
(2) light speed in water is 95% of c;
(3) oscillating electric currents generate radio waves;
(4) radar can measure speed using the Doppler effect?
[UPSC CSE Prelims, 2002]
(A) 1 and 2
(B) 1 and 3
(C) 2 and 4
(D) 3 and 4
Answer: (D) 3 and 4
Explanation: Small molecules preferentially scatter shorter wavelengths, so statement 1 is false. Light in water travels much slower than 95% of vacuum speed, so 2 is false. Oscillating charges generate radio waves and Doppler radar detects motion, making 3 and 4 correct.
