UPSC Prelims 2002 Q69: Science & Technology – Physics
Consider the following statements:
1. Light of longer wavelength is scattered much more than light of shorter wavelength.
2. The speed of visible light in water is 0.95 times its speed in vacuum.
3. Radio waves are produced by rapidly oscillating electrical currents.
4. To detect over - speeding vehicles, police use the Doppler effect of reflected short radio waves.
Which of these statements are correct?
- (a) 1 and 2
- (b) 1 and 3
- (c) 2 and 4
- (d) 3 and 4
Answer: (d) 3 and 4
Explanation
Statements about light and radio waves
| Statement | Verdict |
|---|---|
| 1. Longer wavelength is scattered more | False — shorter wavelengths scatter more (Rayleigh, 1/λ⁴) |
| 2. Speed of light in water is 0.95 times that in vacuum | False — about 0.75 times (refractive index 1.33) |
| 3. Radio waves come from rapidly oscillating electric currents | True |
| 4. Police use the Doppler effect of reflected short radio waves | True — radar speed guns |
Rayleigh scattering is stronger for shorter wavelengths, which is why the sky is blue. Light in water travels at about 0.75 of its speed in vacuum. Radio waves are produced by oscillating currents, and police radar uses the Doppler effect. So statements 3 and 4 are correct.
Answer: (d) 3 and 4.
Why each option is right or wrong
- (a) Incorrect. Statements 1 and 2 are both false.
- (b) Incorrect. Statement 1 is false.
- (c) Incorrect. Statement 2 is false.
- (d) CORRECT. Statements 3 and 4 are true.
Key point
Rayleigh scattering ∝ 1/λ⁴; speed in a medium = c/n; radar guns use the Doppler shift of reflected waves.
Remember
Light in water travels at about three-quarters of c.
Common mistakes
- Thinking red light scatters most.
- Using 0.95 for water instead of 1/1.33.
- Dropping the Doppler application of radar guns used by police.
Exam tip
Two statements have the wrong direction or number; the practical ones are true.
Sources
- NCERT Class X Science — Light
- NCERT Class XII Physics — Wave optics