The name comes from Christian Doppler (1803-1853) who studied the effects motion had on sound. He observed distortions in the pitch of sounds whenever the sound source was moving toward or away from him. He also noticed the same effect if the source of the sound was stationary, and he was in motion relative to the source.
Basically sound is a wave (of compressions) which travels through air at about 1000 feet per second. Each pitch is associated with a certain wavelength or frequency (number of waves that pass by your ear every second). The higher the frequency, the higher the pitch.
A tuning fork making sound (animation)
Click here to hear a tone frequency of 1000 hertz (waves per second).
Click here to hear assorted frequencies.
Now imagine you approach (rapidly) this tuning fork. In the process, your ears will intercept slightly more waves per second, and you will be tricked into hearing a slightly higher pitch. If you move rapidly away from the tuning fork, your ears will perceive a slightly lower frequency, and the tuning fork registers with a lower pitch. You observe the exact same effect if you remain stationary and the tuning fork moves.
First see the Doppler Effect. There are several great web pages that show this effect. Try a few of the following:
Doppler Effect Demo 1 This one needs no directions to use.
Doppler Effect Demo 2 A Java Applet
Doppler Effect Demo 3 (Begin with "start emission", then drag either the source (S) OR the observer (O) to view)
Now hear the Doppler Effect.
Click here to actually hear the Doppler effect as a train passes by. Notice the distinct drop in pitch as the train first approaches and then suddenly receded from you.
This audio clip shows the Doppler Effect as a car zooms by.
Since electromagnetic radiation is also a wave, the Doppler Effect can also be observed. Police often catch speeders by sending out a radio wave which reflects off your car. The reflected wave will exhibit a Doppler shift in frequency if it is moving. The faster the car moves, the larger the frequency shift. To illustrate this effect, consider the animations below. I've used visible light (green) as the incoming radiation. If the car isn't moving, the reflection has the same frequency (green).
 The car is not
moving (animation) |
 The car
approaches (animation) - This is a Doppler
blue shift. |
 The car is
receding from the beam (animation). This is a Doppler
red shift. |
However, if the car approaches the incoming beam, it is reflected with a slightly higher frequency. Since the blue end of the spectrum represents higher frequencies, this is known as a Doppler blue shift. In reality, the car would have to be moving near the speed of light to notice such a large change in frequency. The last animation shows that a car receding from the incoming beam would reflect a beam with a slightly lower frequency, causing a Doppler red shift.
Since every star has an absorption spectra, the Doppler Effect can be put to great use by examining shifts in the location of these absorption lines.
Astronomers can easily tell if a star is approaching or receding by noticing the amount of Doppler shift the absorption line exhibits. They can then convert that amount of shift into a velocity. Notice that this only tells you the speed of the star in your line of sight, known as the radial velocity. It does not tell you how far away it is, or if it is moving perpendicular to your line of sight. Consider the diagram below where a star is moving in space in three possible directions, but with the same speed in each case. Observer of "A" would record a huge Doppler blue shift. Observer of "B" would record no shift at all. Observer of "C" would record a slight Doppler red shift.
Doppler shift only tells you radial velocity.
Oddly enough, most stars have a binary companion (like the twin suns of Tatooine). As the stars dance around each other, they are constantly changing their speed and direction. If the orbit of these stars lies in our line of sight, the spectral lines will shift over time - alternating between Doppler red and blue shifts. With careful observations, astronomers can look at a single point of light and determine how many stars are involved, many of the orbital properties of the system, and even the masses of the individual stars .... all thanks to the Doppler Effect.
If you recall, astronomers claim to have found many exoplanets orbiting other stars. As a planet revolves around the star, it produces small perturbations on the parent star, causing it to "wobble" slightly. Astronomers detect this wobble by noticing slight Doppler shifts in the spectral lines. That is, they really don't see the wobble at all, only detect it in small oscillations in the radial velocity of the star the planet orbits. That is why this method of planetary detection is known as the radial velocity method.
Edwin Hubble claimed that the universe is expanding. Can you guess what tool he used to make this claim?
ŠJim Mihal 2004, 2014- all rights reserved