The brightness of a lightning event was seen at 19:06:17 hr, whereas the sound of the thunder in the same process was heard at 19:06:23 hr. If the speed of the sound is 320 metres per second, then which of the following is the distance in metres between the clouds that produced the lightning and the thunder, and the observer?
- A.1720
- B.1820
- C.1920
- D.2020
Correct answer
Explanation
The correct answer is C, 1920. The thunder arrived 6 seconds after the flash, and sound covers 320 m every second, so the distance is 320 × 6 = 1920 m.
Lightning and thunder are produced at the same moment. Light travels at about 3 lakh kilometres per second, so the flash reaches the observer almost instantly and its travel time can be ignored. Sound in air is far slower, roughly 330 to 340 m per second (the question uses 320 m per second). The gap between seeing the flash and hearing the thunder is therefore the time sound takes to travel from the cloud. Here the gap is from 19:06:17 to 19:06:23, that is 6 seconds, and distance = speed × time gives 1920 m.
A is wrong: 1720 m would need a gap of about 5.4 seconds.
B is wrong: 1820 m would need a gap of about 5.7 seconds.
D is wrong: 2020 m would need a gap of about 6.3 seconds.
Exam tip: We see lightning before we hear thunder because light is far faster than sound; distance = speed of sound × time gap.
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