Doppler!

The Doppler Effect

First, some animation:

http://www.lon-capa.org/~mmp/applist/doppler/d.htm

http://falstad.com/ripple/








So above, the blue dot is emitting sound and moving to the right.  Since it is moving AND emitting sound at the same time, the waves are getting closer on the right - resulting in a shorter wavelength (or higher frequency).  And it is the complete opposite on the left.






http://falstad.com/ripple/
Play with this and choose the "Doppler Effect 1" example.

And for some more visuals:

https://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/78778/Doppler_Nav.swf::Doppler%20Shift%20Interactive


The key in the Doppler effect is that motion makes the "detected" or "perceived" frequencies higher or lower.  This is summarized very nicely in an equation:

It's worth noting that the effect also works in reverse. If you (the detector) move toward a sound-emitter, you'll detect a higher frequency. If you move away from a detector move away from a sound-emitter, you'll detect a lower frequency.

Mind you, these Doppler effects only happen WHILE there is relative motion between source and detector (you).


Does this work for light (and other EM radiation)?  Oh my, yes.

If the source is moving toward you, you detect/measure a higher frequency - this is called a BLUE SHIFT.

If the source is moving away from you, you detect/measure a lower frequency - this is called a RED SHIFT. Distant galaxies in the universe are moving away from us, as determined by their red shifts. This indicates that the universe is indeed expanding (first shown by E. Hubble). The 2011 Nobel Prize in Physics went to local physicist Adam Riess (and 2 others) for the discovery of the accelerating expansion of the universe. Awesome stuff!

http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/





Extra resources:

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