The Doppler Effect
The Doppler Effect is an excellent example of the universality of many discoveries in physics. You can see it in action everyday with passing cars, police sirens, or aircraft just as you can see it in the vast expanses of the universe. The Doppler Effect is the relationship between a wave’s wavelength and the speed of the source emitting it. For example, when a police car with its sirens on is coming towards you, the sirens seem to be a higher pitch than when they are going away. This is because the source of the waves is gets closer to you in between each new crest that is emitted. This means that the wave’s crests are closer together resulting in more waves per second, or a higher frequency. As it goes away, the source is getting further away as each new wave is emitted which results in a lower frequency. This very same principle as you can experience it with sound also occurs with light. If you were to look at distant galaxies in a telescope, they would be shifted towards either end of the light spectrum. While remaining in the visible light part of the spectrum, a body moving toward us would be blue-shifted (a higher frequency) and one moving away would be red-shifted (a lower frequency). So, by looking at the light coming from a body, astronomers are able to tell how fast they are moving towards or away from us. Consider that the further a body is from us, the faster it is moving away. Now you can account for why the sky is dark when we look at the night sky! Because most of the universe is moving away from us so quickly, that the light is shifted towards invisible parts of the light spectrum.
Guest article written by Ryan Wolfe (somescience.tumblr.com)
(Image Credit: 1, 2)