## 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