Question

It was a rainy day in Bangalore. Rohit was sitting besides the window when he saw a lightning. He started his stop watch. He found that after $22$ seconds of lightning its roaring was heard. Calculate the height of the clouds which caused this lightning. Given, speed of light = $3\times {10}^8$ $m{s}^{-1}$ and speed of sound = $340m{s}^{-1}$.

• A. $10km$
• B. $8.5km$
• C. $7.5km$
• D. $5km$

Answer 1

The correct option is C $7.5km$

Speed of light is too high. So you can assume there is no time delay in colliding clouds and lightning.
Since the time taken for light to travel from clouds to earth is approximately 0, we can say that lightning happened at $t=0$ and sound reached our ears at 22 s.
Given, speed of sound $=340m{s}^{-1}$
We know, $distance=speed\times time$
$\therefore$Height of clouds$=22\times 340=7480m\approx 7.5km$.