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Heard a police car go by recently?

Click and listen for the changing sound !

Remember how the siren's pitch changed as the vehicle raced towards, then away from you? First the pitch became higher, then lower. Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves, as illustrated in the following picture.
doppler.gif, University of Illinois

As the police car approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch. As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the ambulance's speed.

 

By analogy, the electromagnetic radiation emitted by a moving object also exhibits the Doppler effect. The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blueshifted.

In contrast, the radiation emitted by an object moving away is stretched or redshifted. As in the ambulance analogy, blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer.

Absorption spectrum, showing blueshift, normal and redshift Scientists know that a cosmic body is approaching us when the identifying lines in its spectral signature appear shifted to the right, or blue side, as compared to that chemical signature seen on Earth. And the converse is true for lines shifted to the left, or red side, are from cosmic bodies that are receding from Earth. When we look at the spectra of a rotating body like a planet or galaxy, we use the Doppler effect to determine which side is approaching us and which is receding from us.

In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Radiation is redshifted when its wavelength increases, and is blueshifted when its wavelength decreases.

The importance of the Doppler Shift is that, by measuring the shifts in wavelength of known spectral features, we can determine such basic information as the masses of stars (using Newton's laws) or the age of the universe (using the Hubble law).
We can even determine the direction in which a star or galaxy is moving. Everything is in motion. Galaxies rotate, and even collide with other galaxies. Our own Milky Way galaxy has collided twice already, and is headed for a collision with our neighbor, the Andromeda Galaxy, in about 3 billion years from now. Collision of galaxies NGC2207 and IC2163

Now, let's see spectroscopy in action! Here comes the sun, little darling, (George Harrison)...