CAPF AC 2021 Q1: Science & Technology – Physics
- (a) About 1650 m
- (b) About 3300 m
- (c) About 990 m
- (d) About 1000 m
Answer: (b) About 3300 m
Explanation
Distance of a storm from lightning and thunder
Given: the time between viewing the lightning flash and hearing the thunder is 10 s.
The speed of sound in air is 330 m/s and the speed of light is 3 × 10⁸ m/s.
How to think: light is so fast that the flash is seen at once.
So the 10 seconds is simply the time taken by the sound to arrive.
| Step | Working |
|---|---|
| Time taken by light | Almost zero |
| Time taken by sound | 10 s |
| Distance | speed × time = 330 × 10 = 3,300 m |
Light and sound compared:
| Wave | Speed in air | Needs a medium? |
|---|---|---|
| Light | 3 × 10⁸ m/s | No |
| Sound | About 330 to 340 m/s | Yes |
Light is about a million times faster than sound.
That is why lightning is seen before thunder is heard.
Answer: (b) About 3300 m.
Why each option is right or wrong
- (a) Incorrect. 1650 m halves the distance, as in an echo; here the sound travels one way only.
- (b) CORRECT. Distance = 330 m/s × 10 s = 3,300 m.
- (c) Incorrect. 990 m would be the distance for a gap of 3 seconds.
- (d) Incorrect. 1000 m does not come from 330 × 10.
Key point
Storm distance = speed of sound × time gap (light arrives instantly). 330 × 10 = 3,300 m. Rule of thumb: about 1 km for every 3 seconds.
Remember
Storm distance = 330 m for every second of delay.
Common mistakes
- Halving the answer as in an echo problem.
- Using the speed of light in the calculation.
- Thinking thunder is produced later than lightning; both occur together.
Exam tip
Educated guess: the speed of light is given only to distract. Multiply the speed of sound by the time.
Sources
- NCERT Class IX Science — Sound (speed of sound in different media)
- NCERT Class VIII Science — Some Natural Phenomena (lightning)