Question

Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. Why?

Answer 1

Hint: Speed of sound is less than speed of light. Light takes a shorter time to travel a distance in comparison to sound. Consider there is a source of sound and light at a specific distance, and determine the time it will take for both of them to travel.

Complete step by step solution:

You have probably experienced the phenomenon of hearing the thunder quite an amount of time after you had seen the lighting. This is one of the most striking examples where you understand the difference in speeds between light and sound.

As you know the speed of light is,
$3\times {{10}^{8}}m/s$
Whereas, the speed of sound is,
$340m/s$
Now, it is very evident from those two values that the speed of light is much more than the speed of sound.

The time required to travel a fixed length of path is inversely proportional to the speed of the wave. Hence, light will take a shorter time to reach your eyes than the sound.

However, the difference in speed is so drastic that the time difference can be quite alarming. We can even assume that, if you have seen the lighting, you can calculate the distance of the origin by measuring the time after which you can hear the lighting.

That is why you can hear a thunder a few seconds after the flash is seen.

Note: We can differentiate between two sounds if the time difference between two sounds reaching our ears is more than 1/10s. So, we can experience echoes if the distance between the wall and the source is more than 17m. So, the total distance to travel will be 34m. Sound will take 1/10 s to travel this distance. However, the echo of light is never possible because of the high speed of light.