Doppler Effect

The Doppler effect is common to all wave phenomena, including light, sound and water waves. When a wave source is moving towards an observer, the source appears to be catching up with the waves as it emits them. They therefore tend to bunch up in front of the source and arrive at the observer with a higher frequency. The reverse effect is noted if the source is moving away. In the case of sound waves, a higher frequency corresponds to a higher perceived pitch; thus the well-known effect of a passing car's dropping engine note as the car goes from approaching to receding.

The same effect occurs when an object reflecting light moves relative to an observer. When approaching, the observed frequency is greater than that being emitted, when receding, it is less. We perceive this frequency shift as a change in the colour of the object. A higher frequency (object approaching) corresponds to a bluer colour, a lower frequency (object receding) to a redder colour. In the example spectrum of a green object shown below, moving towards it would cause the line to shift to the left, causing a spike in the blue region, while moving away from it would shift the spike to the right, placing more energy in the red region.




However, visible light is only a small part of the electromagnetic spectrum. If an observer were to recede from a red-coloured object at a great enough speed, the object would seem to be emitting predominantly infra-red radiation (heat) and would therefore not be visible. Similarly, if the observer were to approach a blue object fast enough, the blue would be shifted into the ultraviolet region and the object would appear darkened.

The following table shows how objects of different colours would appear to an observer if the magnitude of their relative motion was around 25% light speed.

Relative Motion Object colour Apparent colour
ApproachingRedGreen
RecedingRedDarkened (IR)
ApproachingGreenBlue
RecedingGreenRed
ApproachingBlueDarkened (UV)
RecedingBlueGreen