Question

The thunder of a cloud is heard later compared to the flash of lighting because:
A) Speed of sound is greater than speed of light
B) Speed of light is greater than speed of sound
C) Speed of sound is the same as speed of light
D) Sound is obscured by the rain

Answer 1

Hint: In reality lightning and thunder both occur in the same moment but from our perspective it looks like one thing is occurring before the other. Since we are very distant from the point of incident, we feel this way but the same is not felt by us when we are closer to the source.

Complete step by step answer:
The vroom of the thunder is the result of a shock wave produced when lighting super-heats, the atmospheric air through which it passes, as a result air at that point expands and contracts very rapidly. This rapid expansion of air will then create a shock wave to flow outward to the earth atmosphere. One of the reasons we hear thunder rumbles is because sound waves are reflected back and forth from one cloud to another as well.

We already know that the speed of sound at $20\,^\circ C$ is 243 metres per second whereas the speed of light as we know it is $300000$ km per second. If we compare these speeds, light is actually $299792115$ metre per second faster than sound. Due to this ridiculous fast speed of light it takes a fraction of a second for light to reach the observer, us in this case wherein since speed of sound is a lot less and observer distance is significant (few km in this case) it takes a couple of seconds for sound to reach our ears.

Hence option (B) is the correct answer.

Note: Speed of sound is also affected by change in pressure and temperature but since the difference is much greater than both speed, we have taken speed of sound at $20\,^\circ C$ to show things in perspective.