Can anyone explain the Doppler effect to me?
Think of an ambulance with its siren on, coming down the street towards you. As it passes you, the pitch of the siren is lower than it was before it passed you. That is an example of the Doppler effect. When light waves or sound waves are emitted by an object, they radiate out in all directions. If the source is moving fast enough (in relation to the wavelengths), then the waves will be bunched up in front of the object and spread out behind it. The bunched up waves will be at higher frequencies (in sound, the higher frequencies are higher pitched to our ears, in light the higher frequencies are towards the blue end of the visible light spectrum). Christian Doppler formalized the concept in 1842 (the idea was known to the ancients). There is a lot of information on wikipedia if you really want to know more.
