The Doppler Effect was first proposed in 1842 by Christian Doppler. He was an Austrian physicist and mathematician. He studied the Effect with respect to sound waves.
An easy example of the effect has to do with the sirens on ambulances or police cars. Have you ever noticed how the pitch of the siren changes as it moves toward or away from you? This is the Doppler Effect.
As the vehicle approaches, the sound waves from its siren are compressed toward you. The intervals between the waves diminish. This translates into an increase in frequency or pitch.
As the vehicle moves away from you, the sound waves are more or less stretched. The distance between the waves gets greater. This has the effect of lowering the frequency or pitch. By the change in pitch, we know whether the siren is moving toward us or moving away. If you could calculate the rate of change in the pitch, you could also roughly calculate the speed of the vehicle.
We now know that the Doppler Effect hold for all sorts of waves, not just sound waves. Astrophysicists use the Doppler effect with respect to the electromagnetic spectrum, both the visible and invisible, as they seek to understand elements of the universe.
There is an inverse relationship between frequency and wavelength. In other words, as frequency decreases, wavelength increases. Physicists describe this in terms of redshifting (moving toward the red end of the spectrum) and blueshifting (moving toward the blue end of the spectrum). Objects moving away from a stationary source are said to redshift, while those moving toward a stationary source are said to blueshift.
The Doppler shift is used to calculate precisely how fast stars and other celestial bodies are moving toward or away from the earth. For example, they may observe spectral lines from hydrogen gas in a distant galaxy. This is generally considerably redshifted, meaning that it is moving away from the earth at some speed. Normally on earth, the spectral line emission of hydrogen is at a wavelength of 21 centimeters. If the observation in the distant galaxy is at 21.1 centimeters, they can calculate that the gas is moving away from earth at some 880 miles per second., and thus so is the galaxy.
There are two other factors that impact frequency shift. One is associated with very strong gravitational fields and is called gravitational redshift. The other is concerned with the fact that the universe is expanding and is called cosmological redshift. Both values are known and are taken into consideration is calculations of speed of motion.
A very common application of the Doppler Effect, one that you may hear about every day, is Doppler Radar, which is used in weather forecasting. Essentially, weather radar sends out radio waves from an antenna. Objects in the air such as rain, snow, hail, etc scatter or reflect some of the waves back to the antenna. The reflected radio waves are converted into images showing the location and intensity of the precipitation.
The Doppler Effect comes into play as the computer measures the frequency change of the returning radio waves. It uses the changes to show the direction and speeds of the winds blowing around the precipitation. If the frequency never changes, it is likely caused by so-called ground clutter, most likely trees, buildings, other stationary objects.
Sonic booms, usually produced by airplanes passing through the sound barrier, are another illustration of the Doppler Effect. As the plane approaches the speed of sound, the sound waves are increasingly compressed at the front of the plane. Pilots report a noticeable wall or barrier as they approach the speed of sound, due to this intense compression of the sound waves. Before Chuck Yeager first broke the sound barrier in 1947 in the X-19 rocket plane, no one really knew what would happen to him or the plane when they reached the speed necessary. As the plane approached the magic speed (about 750 miles per hour), it began to shake violently and the noise was unbearably loud. But Yeager controlled it and kept on, and finally broke through.
When the plane reaches the speed of sound, and passes through, it is said to go supersonic. The nose of the plane actually leads the advancing wavefront and is out ahead of it. Yeager immediately experienced a tremendous lessening of the sound once he had passed through. To a stationary observer, the plane will pass by well before the sound it creates does.
The sound is initially caused by the shock wave, which is advancing at the speed of sound, while the plane is going faster. It is a buildup of all of the combined wavefronts that have been compressed. It is quite intense to the observer and initially comes as a sonic boom. When the Concorde was developed, it was the first commercial passenger plane capable of supersonic speed. All sorts of restrictions were put on it and the flight path to prevent the sonic boom from occurring over land, due to the supposed disruption.
There have been high speed photos taken of objects like planes and bullets approaching, then breaking through the sound barrier. In these, the compression of the sound waves are clearly shown in the front, with the concomitant lengthening at the rear, just as would be expected by the Doppler Effect.
It is always amazing how a simple observation like the change in pitch of a siren, or the falling of an apple from a tree, can lead to such breathtaking breakthroughs in physics. In terms of the Doppler Effect, it is one of the defining characteristics of a wave.