Wednesday, November 07, 2007

Physics: How are sound and light different?

Sound and light are very different, even though they are both waves.  For instance, one can travel in outer space and the other one cannot - at least outside of Hollywood, sound doesn't travel in a vacuum! 

To explain more fully, let's start off with how light and sound are the same.  Both are indeed waves and therefore have amplitudes, frequencies, wavelengths, and speeds.  Wavelength is the distance a wave travels before repeating, or, put another way, it is the distance between two successive troughs or peaks on a periodic (repeating) wave.  Frequency is the number of times a wave repeats in a second and is closely (and inversely) related to wavelength.  The speed of a wave is the frequency times the wavelength.

One way that they differ is in how they propagate. Sound requires a media to travel in because it is basically just a change in pressure.  For example, in air, it is a change in air pressure that carries the sound.  Sound can also be carried by other media, such as water (e.g. sonar).  Light, on the other hand, is an electromagnetic wave, which means that it is a combination of electric and magnetic fields that travel together.  Therefore, it does not need air, water, or any other medium to carry it - in essence, it carries its medium with it.

Another way in which they differ is in their speeds.  Sound travels about a foot (approximately a third of a meter) in a millisecond.  Light, on the other had, travels 186,000 miles (300,000 kilometers) per second.  Talk about the tortoise and the hare!

So, despite many similarities, sound and light differ in ways that are significant and have consequences for many different life forms - but that is a topic for another posting.

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