ELi5: Why does the time between lightning striking and when you hear the thunder correlate to how far the lightning landed?
In: Physics
Light travels way faster than sound. If a lightning bolt is far away, the light will take a negligible amount of time to reach your eyes, but the sound will take really long in comparison. The further away the lightning is, the bigger this gap will be because of just how much faster light is than sound.
Because light and sound travel at different speeds.
You see the lightening flash (almost) instantaneously, but it takes the sound longer to reach your ears.
If you can time how long between see the flash and hearing the thunder, you can calculate how far away it is.
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Sound travels slower than light, by a huge amount. Sound needs about five seconds to travel a mile. Light only needs 0.000005 seconds to travel that same mile. So the light from the lightning gets to you right away, but the sound won’t get to you for five seconds, if the lightning struck a mile away. Count the seconds from the flash to the bang and for every five seconds, the lightning was a mile away.
Sound travels through different mediums at relatively constantly speeds. The same can be said for light. You see the lightning first because light travels faster than sound. How far ahead the light is tells you how far the lightning is. If it is a mile away, the thunder will arrive 5 seconds later
It comes down to Distance = Speed x Time.
Sound and light do no travel at the same speed. The speed of sound is about 343 metre per second (m/s), while light travels at 3 * 10^8 m/s—it’s a big difference. If you are close enough to an object, you will see it and hear it at the same time. However, if the object is far away, you will see it first, then it will take some time for that sound to reach you.
If you see a flash, then it takes a full second to hear a noise, that means the flash was 343 metres away. If you see a flash of lighting, count to three, then hear the thunder that means that you are 3 x 343m away, which is 1029 metre, or about kilometre away. As a rule of thumb, every three seconds is kilometre, and every five seconds is a mile.
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