Question

Flash and thunder are produced simultaneously. But thunder is heard a few
seconds after the flash is seen, why? How does the speed of sound in air vary with
rise in density of the medium?

Answer 1

Flash and thunder are produced are produced simultaneously,but the flash is seen first and then the sound is heard.This is because the speed of sound in air is very less than the speed of light in air.
Speed of sound in air : 334ms⁻¹
Speed of light in air : 3X10⁸ms⁻¹

With the increase in Density of the medium, the speed of sound also increases.

Answer 2

In lightning process, flash and thunder are produced simultaneously. Flash is seen almost immediately because speed of light is extraordinarily large. But thunder is heard a few seconds later because speed of sound << speed of light.