Doppler Effect is the phenomenon of change in the frequency of a wave due to relative motion of the wave source and the observer. This is easily observed in a highway where the siren of moving police vehicles or ambulances tend to get high pitched when they are getting closer and closer and the other way around when they are moving away.

When the source and the observer moves away or towards relatively, the wave fronts from the source either get separated or cramped together. This results in a change, in the rate of wave fronts received by the observer than the rate it was emitted by the source. Since this rate is recorded as the frequency, the source’s frequency and the apparent frequency are different. Doppler Effect can be observed in every wave, whether electromagnetic or mechanical.

When the source and the observer are moving relatively towards each other, then the apparent frequency is higher than the source frequency. If the source and the observer are receding relative to each other, then the apparent frequency is lower than the source frequency. Since frequency change is related to the motion of the observer and the source, it can be used to deduce the motion.

Assume the observer is stationary. If the apparent frequency is higher than the source frequency, it can be deduced that the source is moving towards the observer. If the apparent frequency is lower than the source, then the source is moving away.

In case of light, the relative motion of the source and observer causes the frequency to shift either in the direction of the red colour or the blue colour. If the light has moved in the direction of the red, the objects are relatively moving away, and they are said to display a redshift, and a blue shift is when moving towards each other. In fact, this is observed first when trying to determine the spectral types of stars.

Redshift and blueshift are shift in the observed frequency of visible light due to relative motion of the source and the observer.

• For redshift, the sources and the observer are relatively moving away from each other, and the Z value is positive.

• For Blueshift, the source and the observer are moving towards each other, and the Z value is negative.

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