When wave energy like sound or radio waves travels between two objects, the wavelength can seem to be changed if either one or both of them are moving. This is called the Doppler effect.
The Doppler effect causes the received frequency of a source (the way it is perceived when it gets to its destination) to differ from the initial frequency if there is motion that is increasing or decreasing the distance between the source and the receiver. This effect is readily observable as a variation in the pitch of a sound between a moving source and a stationary observer.
Imagine the sound a ringing car makes as it rushes by, whining high pitched and then suddenly lower noises. Vrrrm-VROOM! The high pitched whine is caused by the sound waves being compacted as the car approaches you, while the lower pitched VROOM comes after it passes you and is speeding away. The waves are spread out.
When the distance between the source and the recipient of the electromagnetic waves remains constant, the frequency of the waves is the same in both places.
When the distance between the source and the recipient of the electromagnetic waves is increasing, the frequency of the received wave form is lower than the frequency of the source wave form. When the distance is decreasing, the frequency of the received wave form will be higher than the source wave form.
Besides sound and radio waves, the Doppler effect also affects the light emitted by other bodies in space. If a body in space is 'blue shifted', its light waves are compacted and it is coming towards us. If it is 'red shifted', the light waves are spread apart, and it is traveling away from us. All other stars we have detected are 'red shifted', which is one piece of evidence for the theory that the universe is constantly expanding, perhaps from a 'Big Bang'.
