The Doppler Effect

train tunnel painting

Spectroscopy’s most dramatic application was in determining an object’s motion towards or away from earth. In 1868
William Huggins found that dark lines in the stars, Sirius, were shifted towards the red end of the spectrum. It was based on the work of an Austrian physicist, Christian Doppler, who held a theory that a moving source would change its  direction, i.e whether it approaches the observer or moves away.

This can be illustrated by a simple example, to a stationery observer on a platform, the whistle of a train moving away from him will’ fade’ away.

This is because the sound waves are lengthened and the pitch becomes lower and lower. On the contrary, when the train approaches the observer, its whistles become shriller, as the sound waves are shorted resulting in a higher pitch.

train tunnel painting

Doppler thought that the same effect could be experienced with light as well: the longer the light wave, the nearer it will move to the red end the spectrum. Light from an object approaching the observer would be blue shifted, white it would be red shifted, if the object is moving away. In fact, the Doppler shift applies to all electromagnetic radiation;.

Doppler first thought that the red and blue shifts were causing the differences of colours observed in some binary stars however, the movement actually showed the shift of the spectral lines in light coming from a star , shifting towards the red or blue , i.e. moving away or approaching the observer.

A measure of the shift would reveal how rapidly the star is approaching or receding. Velocity of the recession can thus be calculated from the shift of the lines.The Doppler technique is quite useful in finding out the size, period and eccentricity (deviation from a perfect circle) of a planet’s orbit.

Practical Applications of the Doppler Effect

Since it’s discovery more than 150 years ago the Doppler effect has found it’s application in many devices . For instance the radar gun used by the Traffic police for checking the speed of vehicles relies on the Doppler Effect.

The Doppler effect is also used by radiologists to produce echo-cardiograms for patients.

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