The Doppler Effect
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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 (in optical light, this moves the light to the BLUE):
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), in optical light, this moves the light to the RED:
Most importantly, the stronger the Doppler shift, the faster the object is moving towards or away from you. So measuring the amount of the shift also tells you the speed of the object.
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.
Click
Here to see a
movie of an object traveling at Mach speed.
Click
Here to see a
movie of an object traveling at Mach 1.4, faster than
the wavespeed.
