What is the doppler effect?

The Doppler effect is caused when waves (such as light or sound waves) are emitted from a moving source which causes an increase or decrease in the wavelength of the emitted wave. This increase or decrease depends on whether the source is moving towards or away from you, movement towards you causes a decrease in wavelength whilst movement away causes an increase. Examples include the redshift of light from distant stars moving away from us and the change in pitch of the sound a car makes as it drives past.