Question

It was a rainy day in Bangalore. Rohit was sitting beside the window when he saw a lightning. He started his stopwatch. He found that after 22 seconds of lightning its roaring was heard. Calculate the distance of the clouds which caused this lightning. Given,
Speed of light = 3 x ${10}^8$ $\frac{m}{s}$ and Speed of sound = $340\frac{m}{s}$.

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

Answer 1

The correct option is C

7.5km

The speed of light is too high. So you can assume no time delay in the occurrence of lightning and Rohit observing it. Since the time taken for light to travel from clouds to Rohit is approximately 0.

So we can say that lightning happened at t=0, and sound reached his ears at 22s.

Therefore the distance of the clouds = speed of sound $\times$ Time taken = 340 $\times$ 22 =7480m ≈ 7.5km.