When a vibrating source of waves is approaching an observer, the frequency observed is higher than the frequency emitted by the source. When the source is receding, the observed frequency is lower than that emitted. This is known as the Doppler effect, or Doppler’s principle, and is named after an Austrian physicist who lived in the first half of the 19th century.

When a whistling locomotive (or any other sound source) approaches a stationery observer (Fig.1), more density concentrations reach his ear than when both the sound source and the observer are stationary.

As the pitch depends on the frequency (number of vibrations per second), the sound from the approaching locomotive’s whistle has a higher pitch than the sound coming from the same whistle when the locomotive is stationary in relation to the observer.

When the locomotive is receding, its whistle sounds with a lower note. At the instant when the locomotive passes the observer, the note of the whistle is heard to change to a lower pitch. The same effect is observed when we are passed by a fast-moving hooting car in the street, or when the observer is moving fast in relation to a stationary sound source. E.g., a motor cyclist approaching a siren (Fig.2).

The Doppler effect is widely used in astronomy for measuring the velocity at which distant stars or nebulae are approaching or receding. These motions produce a shift in the position of lines in their spectra.

A particular spectrum line corresponds to a certain definite light wavelength. If the star emitting the light is moving away from us, its light rays have a longer wavelength (lower frequency) by virtue of the Doppler principle, and this is manifested in a general shift of the spectrum lines towards the red end of the spectrum. This is known as the red shift.

In the spectrum of a star moving towards us, the characteristic lines would show a blue shift, i.e., they would be displaced towards the blue end of the spectrum, corresponding to shorter wavelengths and higher frequencies. These phenomena are indicated in Fig.3

A Remarkable thing about the spectra of the spiral nebulae (the galaxies of stars far out in space beyond our own milky way system) is that they all display the red shift and must therefore on the basis of Doppler’s principle all be moving away from us.

The theory of the expanding universe is based on this phenomenon. However, this interpretation of the red shift is disputed by some authorities.

This is one example by which Doppler Effect can be understood very easily. An ambulance Siren go by recently. 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.

As the ambulance 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.