When the radio makes sound, it spreads out in all directions in the form of waves. The air around it is compressed, shown by the blue circles, and the decompressed, shown by the white areas in between. These waves move away from the radio at the speed of sound, which is roughly 1220 kilometers per hour: pretty fast.

	At that speed, it seems to the girl as if the sound reaches her ear as soon as she turns on the radio, but it actually takes a tiny fraction of a second for the first wave to reach her ear.
	Once the first wave arrives, they keep coming.
	The sound waves reach her ear with the same frequency as they left the radio. Let’s say that there’s a trumpet playing the note A above middle C, which is 440 hertz, or 440 cycles per second. That means that 440 waves per second leave the radio and, after a short interval, they start arriving at the girl’s ear at the rate of 440 per second. So she hears the proper note.

But what if she’s moving toward the radio? Since she keeps shortening the distance between the radio and her ear, each wave has a shorter distance to travel than the one before and arrives at her ear a bit earlier than it would if she were not moving. The result is that slightly more than 440 waves reach her each second so the sound has a slightly higher pitch than the sound produced by the radio. If she moves away, then each wave takes a bit longer to reach her and the pitch is slightly lower.

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What does this have to do with the expansion of the universe?

		Digging Deeper
		Find out more about
	the Doppler effect.

The Doppler Effect applies to light as well as to sound. The white light coming from the sun and from distant stars is actually made up of all the colors of the rainbow, from the lower frequency reds and oranges to the higher frequency blues and purples. If you and a light source are approaching each other the pattern of colors that make up the light, known as the spectrum, is shifted towards the blue end. If you and the light source are moving away from each other, the shift is towards the red end. In the 1920s Edward Hubble discovered that the light from distant stars and galaxies was red shifted more than from nearby light sources. This confirmed that the universe was expanding. To find out more about this read The Expanding Universe.