Question

Distance from a lightning strike A general rule of thumb is that if you see lightning, there is one mile for every five seconds before the thunder is heard. Check this against the actual speed of sound Speed of sound at 20 C is approximately 343 m/s.

Answer 1

The time taken by light to travel 1 mile can be calculated by

$t_l=\frac{d}{v}=\frac{1\ mile}{3\times 10^8\ m/s}=\frac{1\times 1609.34\ m}{3\times 10^8\ m/s}=5.36\times 10^{-6}\ s$

Now, the time taken by the sound to travel 1 mile is

$t_s=\frac{d}{v}=\frac{1\ mile}{343\ m/s}=\frac{1\times 1609.34\ m}{343\ m/s}=4.69\ s$

From our calculation

$t_s>>t_l$

That gives us

$t_s-t_l\approx t_s=4.69\ s \approx 5\ s$

So, the statement is good up to some approximation.