The Doppler Effect

Have you ever noticed the way the pitch of a train whistle changes as it rushes by you? This is called the Doppler Effect.

The sound of the whistle travels to you via a sound wave. If the train were standing still the whistle would have a certain pitch, determined by the length of the sound waves. As the train moves toward you, the whistle emits each part of the wave a little bit closer to you that the last part. The length of the waves appears shorter, and you hear a higher pitch. After the train passes you , the whistle emits each part of the wave a little bit farther from you than the last part. This makes the length of the waves appear longer, and you hear a lower pitch.

A similar effect happens with light. However, with light, pitch is replaced by color. "Higher pitch" light is bluer, "lower pitch" light is redder. Therefore, when a light source is traveling away from us, we say it is "red-shifted", and when it is traveling towards us, we say it is "blue-shifted".

We can quantify how much the wavelength of the light will change:

Wavelength change = Original wavelength * (Velocity of the Source / Speed of Light)

Want to see this in action? Try out the Doppler-Tron 2000 below. What happens to the length of the waves as the sources moves faster and faster towards you? away from you? What about the color of the waves?

Doppler-Tron 2000