Speed of light vs speed of sound
Light is about 874,000 times faster than sound in air, which is why the delay exists.
Weather & Physics
Why Does Thunder Come After Lightning?
Thunder is not a separate thing that happens after lightning. It is the sound of lightning-heated air expanding, but sound reaches you much later than light.
The short answer
Thunder and lightning are not separate events. Lightning superheats the air around it to roughly 30,000 Kelvin in less than a millisecond. That air expands explosively and creates a pressure wave. That pressure wave is thunder. You see the flash first because light travels at about 300,000 kilometers per second, while sound travels at about 343 meters per second near sea level. A lightning flash one kilometer away reaches your eyes almost instantly. Its thunder takes about three seconds to reach your ears. That delay is useful. Count the seconds between the flash and thunder, then divide by three for kilometers or by five for miles. The shrinking delay tells you the storm is moving closer.
How to calculate storm distance
Count seconds between flash and thunder, divide by 3 for kilometers or 5 for miles.
Myth: if you hear thunder, you are safe from that bolt
If you can hear thunder, you are within range of future lightning from the same storm.
What makes thunder
Thunder is the shock wave from air suddenly heated and expanded by the lightning channel.
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