How Police Radar Works

The Doppler effect.

http://www.physicsclassroom.com/class/waves/u10l3d.cfm
That little dude is about to get nailed.

Radio waves are sent from a antennae, be it a stationary one on the front and back of a police vehicle or from a gun, at certain frequency in one of 13 channels from 33.4 to 36.0 GHz  range.  (Aside: Many garage door openers are in the 300-400 MHz range. 18–24 GHz is strongly absorbed by water vapor because of the natural resonance of that molecule.  It is, therefore, used in meteorology.  This used to be the frequency often used by the police, but it was abandoned because of the interference during high humidity days.)  The beam strikes the vehicle and the reflection speed of the vehicle causes the wave length to increase (or decrease if it's driving away).  Measure the slight difference in wavelength and you can calculate the speed of the vehicle.  Now, an indirect bounce off the vehicle might skew the reading, but that would only cause it to appear slower.

This same Doppler effect is how we know the universe is expanding--everything is shifted to a slower, redder frequency.  Meaning, it's pretty much all moving away from us.  If you project the speed of the expansion based on that red shift and then project backwards how long the universe has been expanding you get a rough estimate of the age of the universe since the Big Bang (requiring adjustment for dark energy).

http://en.wikipedia.org/wiki/Redshift  

http://www.psychedelicporcupine.co.uk/2010/02/doppler-effect-and-red-shift/

This all sounds simple enough...but don't forget that light isn't like sound.  In sound the compression and rarefaction waves are just sent further or closer together.  No big deal.  That makes sense.  Molecules just get closer or further apart.  What about light is 'compressed' or 'stretched'???  It ought not sit so easily with you if you think about it.  Light is a particle.  No it's not.  It's a wave.  Well, what exactly is 'waving'?...