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Why do we see the lightning before hearing the thunder?

Why do we see the lightning before hearing the thunder?
Elizabeth Tatham from Greater London (Age 5-14)

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4 Responses

  1. This is to do with the difference in the speed of light and sound. To see or hear something we need the light or sound from that object to reach our eyes or ears. Light waves travel very quickly at the speed of approximately 670,000,000 mph. At this speed, you can say that they reach you almost instantaneously. Sound waves are much slower, covering only about 770 mph. For a thunderstorm that happens 1 mile away, it will take about 4 and a half seconds for the sound to reach you. The longer the pause between the lightning and the thunder, the further away the thunderstorm is.

  2. I don’t know if you know this, but light and sound travel in waves. Light waves travel at roughly 300,000,000 metres per second, and sound waves travel at about 340 metres per second.
    This huge difference in speed means that the light from lightning gets to us quicker than the sound of thunder does. This is why the furthur away the lightning is, the longer wait between when you see the lightning and hear the thunder.

  3. Dear Elizabeth

    Let’s start by getting one thing straight – the thunder is just the big noise that the lightning makes!
    So, if you were right next to a lightning strike (hopefully you never will be!) then you would see and hear the lightning at almost exactly the same time.
    However, the speed of sound is pretty slow compared with the speed of light. So if you are far away from the storm (much more likely) then you still see the lightning right away (because light is really fast) but it takes some time for the sound to reach your ears (because sound is fairly slow).
    In fact, it takes about 3 seconds for every kilometre the sound has to travel.
    So, if you count the seconds between the flash and the thunder, then it tells you how far away the storm is. Count 3 seconds, and its 1km away. Count 6 seconds, and its 2km away.
    This is neat, because it even lets you know if the storm is coming closer to you, or moving further away. If you start counting longer delays, then the storm is moving away. However counting shorter and shorter delays means the storm is getting closer.
    Hope this helps!

  4. Also I think the light from the lightening flash travels to our eyes much faster (around one million times faster) than the sound of the thunder associated with the lightening flash travelling to our ears.

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