Doppler Effect for Moving Objects
A Red Shift or Blue Shift in Light and a Pitch Change in Sound
The Doppler effect works for both light and sound. In light the wavelength of a spectral line changes. For sound the pitch changes.
Sound
Listen carefully to the pitch of the siren on a police car or emergency vehicle as it passes you. Be sure to distinguish between the pitch or frequency of the sound and the volume. When the siren is moving towards you, it has a higher pitch. After it passes, it has a lower pitch when it is moving away. The effect is greater at faster speeds. This effect is called the Doppler effect.
When the source of sound is moving towards the listener, the sound waves are squeezed. Hence they have a shorter wavelength. When the source is moving away, the sound waves are stretched out to longer wavelengths. For sound waves, a longer wavelength and lower frequency corresponds to a lower pitch. A shorter wavelength and higher frequency produces a higher pitch sound.
The effect is the same whether it is the source of the sound or the listener that is moving. It is the relative motion that counts here rather than which object is moving. Therefore if either the sound source or the listener are moving towards each other the listener hears a higher pitch sound. If they are moving away from each other, the sound is a lower pitch. With care you can hear the pitch change as the siren passes you.
Light
This Doppler Effect works for light and other electromagnetic radiation as well as sound. If either the light source or observer are moving towards each other, the light waves are squeezed to a shorter wavelength. If they are moving apart the waves are stretched to a longer wavelength.
For visible light, red light has a longer wavelength than blue light. A shift to a shorter wavelength is therefore called a blue shift, and a shift to a longer wavelength is a red shift. As for sound the amount of the red or blue shift is greater when the motion is faster.
Some Applications
Astronomers use the Doppler effect to tell if stars or other celestial objects are moving towards us or away from us and how fast they are moving. They look at the wavelengths of the spectral lines and compare them to the wavelengths of the same spectral lines measured in a laboratory on Earth. For example Hubble discovered that the universe was expanding by measuring the red shifts of very distant galaxies.
If you have gotten a speeding ticket recently, the officer likely used radar to measure your speed. Police radar devices send out radio waves that reflect off a car back to the radar device. If the car is moving, the radar device measures the change in the radio wave's wavelength. The greater the change, the greater the speed. The computer in the radar device corrects for the speed of the police car.
Weather reports often make use of Doppler weather radar. The radio wave reflects off a cloud or storm system. The wavelength change tells the meteorologists how fast the storm is moving.
