CAPF AC 2020 Q5: Science & Technology – Physics
- (a) Raman effect
- (b) Induction effect
- (c) Doppler effect
- (d) Coulomb effect
Answer: (c) Doppler effect
Explanation
Police speed radar and the Doppler effect
How to think: the radar must find the speed of a moving vehicle.
Only one effect in the options depends on motion.
The Doppler effect is the change in the observed frequency of a wave when the source and the observer move relative to each other.
A speed gun sends radio waves (microwaves) at a vehicle.
The waves reflect off the moving vehicle with a changed frequency.
The size of the change gives the speed of the vehicle.
| Option | What it is | Verdict |
|---|---|---|
| (a) Raman effect | Change in wavelength of light scattered by molecules | Incorrect |
| (b) Induction effect | A changing magnetic field produces a current | Incorrect |
| (c) Doppler effect | Frequency changes with relative motion | Correct |
| (d) Coulomb effect | Force between electric charges | Incorrect |
Other uses of the Doppler effect:
| Use | Detail |
|---|---|
| Doppler weather radar | Tracks moving rain and storms |
| Echocardiography | Measures blood flow in the heart |
| Astronomy | Red shift shows galaxies moving away |
Answer: (c) Doppler effect.
Why each option is right or wrong
- (a) Incorrect. The Raman effect is about light scattered by molecules; it is used to study materials.
- (b) Incorrect. Induction produces a current from a changing magnetic field; it does not measure speed.
- (c) CORRECT. The radar reads the frequency shift of waves reflected from the moving vehicle.
- (d) Incorrect. Coulomb's law gives the force between charges, unrelated to radar.
Key point
Doppler effect: observed frequency changes with relative motion. Used in police speed radar, weather radar, echocardiography and red shift.
Remember
Speed radar = Doppler effect (frequency shift of reflected waves).
Common mistakes
- Picking the Raman effect because it is also about a change in frequency of scattered light.
- Thinking the Doppler effect applies only to sound; it applies to radio waves and light too.
- Forgetting that a siren sounds higher when approaching and lower when moving away.
Exam tip
Educated guess: Raman is about scattered light, induction about current, Coulomb about charges. Only Doppler is about motion.
Sources
- NCERT Class XI Physics — Waves (Doppler effect)
- NCERT Class XII Physics — Electromagnetic Waves (radio waves and microwaves in radar)