Page 7, Properties of the Stars II: [Previous Page] [Next Page] [Class Home Page] Text Pages 95-97

Light is a wave, and as a wave, it experiences the Doppler Effect. Sound is also a wave. We have all experienced the sound of a whistle on a train or siren on a vehicle, and the change in pitch it undergoes at it passes by us.

When we approach a wave, we INCREASE the speed with which the crests pass us, there by increasing the frequency, or shortening the wavelength:



When we move away from a wave (or the source emitting the wave moves away from us), we DECREASE the speed with which the crests pass. This decreases the frequency (increases the wavelength):






Mathematically, the Doppler Effect is described in this way:

Lambda is the wavelength of the light, delta-lambda represents how much the wavelength has shifted, and c is the velocity of the wave.

You can see from this equation, unless the velocity between wave emitter and wave `listener' is some reasonable fraction of the speed of the wave (c), the wavelength shift will be imperceivable. This is why you can not drive fast enough in your car to make a red stop light turn green!

When the emitting object travels at the wavespeed (`c'), it is said to be traveling at Mach 1. Click Here to see a movie of an object traveling at Mach 0.7, 70% of wavespeed. In front of the object, the distance between waves is much smaller than in back of the object.