Q1. When light enters a closed room through a small hole in the door, the image of an outside building appears as inverted on the opposite wall. This is because
(a) The hole acts as a convex lens
(b) Light takes curvature at the edges of the hole
(c) Of rectilinear propagation of light
(d) The hole acts as a concave lens
(UPSC Prelims 1979)
Answer: (c) Of rectilinear propagation of light
Explanation: Light travels in straight lines, a principle known as rectilinear propagation. When it passes through a small hole, it forms an inverted image on the opposite wall, similar to a pinhole camera, due to the crossing of light rays.
Q2. What are the primary colours ?
(a) Blue, green, yellow
(b) Yellow, red, blue
(c) Red, blue, green
(d) Green, white, black
(UPSC Prelims 1981)
Answer: (c) Red, blue, green
Explanation: The primary colours of light are red, blue, and green. These are the basic components of the additive colour model, used in digital screens and optics. When combined, they produce white light and various other colours.
Q3. In a fluorescent tube which of the following components are found?
(a) Mercury vapour
(b) Argon and carbon dioxide
(c) Helium and oxygen
(d) Argon and neon
Answer: (a) Mercury vapour
Explanation: A fluorescent tube contains mercury vapour, which emits ultraviolet light when electrically excited. This UV light then interacts with the phosphor coating inside the tube to produce visible light.
Q4. Cover of a solar cooker is made of glass. Why?
(a) It absorbs sun rays
(b) It reflects sun rays
(c) Sun heat is transmitted through glass without much absorption
(d) None of these
(UPSC Prelims 1984)
Answer: (c) Sun heat is transmitted through glass without much absorption
Explanation: Glass allows solar radiation to pass through and traps the heat inside the cooker, creating a greenhouse effect. This helps in raising the temperature efficiently for cooking. It does not absorb or reflect the rays significantly.
Q5. Radio waves travel with a speed of
(a) 3 × 10⁶ m/sec
(b) 3 × 10⁷ m/sec
(c) 3 × 10⁸ m/sec
(d) 3 × 10⁹ m/sec
(UPSC Prelims 1985)
Answer: (c) 3 × 10⁸ m/sec
Explanation: Radio waves, like all electromagnetic waves, travel at the speed of light, which is approximately 3 × 10⁸ metres per second in vacuum. This is a fundamental physical constant.
Q6. If the distance that a radio wave can travel in the time of one complete cycle is 10 metres, then the frequency of the radio wave in cycles per second is
(a) 3 × 10⁸ m/sec
(b) 3 × 10⁶ m/sec
(c) 3 × 10⁹ m/sec
(d) 3 × 10⁷ m/sec
(UPSC Prelims 1985)
Answer: (d) 3 × 10⁷ m/sec
Explanation:
Speed = Frequency × Wavelength
Given: Speed = 3 × 10⁸ m/sec, Wavelength = 10 m
Frequency = Speed / Wavelength = (3 × 10⁸) / 10 = 3 × 10⁷ cycles/sec
Hence, the correct frequency is 3 × 10⁷ Hz.