Speed of sound and speed of light

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You can learn a lot about speeds of sound and light during a thunderstorm. When you see a flash of lightning across the sky, the rumble comes only a few seconds later. Although both light and sound are actually produced at the same moment, we perceive them separately.

That’s because light travels much faster than sound. In fact, nothing in nature can compare to the speed of light.

To visualise the speed of light, take, for example, sunlight which takes only about 8 minutes 17 seconds to travel to the surface of Earth. The speed of light is only constant in vacuum and equals 299,792,458 m/s or ca. 186,282 mi/s. Light gets slowed down in transparent media, e.g. water, glass, ice, diamond, air etc.

The speed of sound is the distance that a sound wave travels per unit of time. It varies depending on the medium through which the sound travels. The speed of sound is lower in gases than in solids; to compare: ca. 343 m/s in air, 1,484m/s in water and 12,000 m/s in diamond.

The speed of sound differs depending on the temperature, too:

Air temperature (oC)
Speed of sound (m/s)
30
349.02
20
343.21
10
337.31
0
331.30
-10
325.18
-20
318.94


Did you know?

The next time you witness a thunderstorm, you can calculate how far away from the thunder you are thanks to a technique called ‘flash-to-bang’. All you need to do is count the seconds that pass between the flash of lightning and the rumble and divide that number by 5: the result tells you how many miles away from you the thunder had struck.

If you prefer to get the result in metres, multiply the number of seconds between the flash and the crack by 340, as light travels at ca. 340 m/s.

Have you heard about this technique before?

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