An observer heard the sound of thunder 5.8s after the lightning flash was seen. How far was he from the source? [Speed of sound in air = 330$ms^{-1}$]

Analyze the question: Light travels so fast that we see the lightning almost instantly. The sound travels at 330 m/s, so the 5.8 s delay is essentially the time the sound took to reach the observer.

Concept: Distance = speed × time

Solution:

• Speed of sound, v = 330 m/s
• Time delay, t = 5.8 s

d = v × t d = 330 × 5.8 d = 1914 m

Check against the options: 1914 m matches option B. (Option A, 957 m, is roughly half, which would come from wrongly dividing by 2, as you would for an echo. Thunder is a one-way trip, so no halving is needed.)

Tip: Watch the wording. For thunder and lightning, the sound travels one way, so use d = v × t. For echoes, the sound travels to the obstacle and back, so use d = vt/2. Many students lose marks by mixing these up.