# The Doppler Effect Blue Shift and Red Shift

Let us say that light waves travel from a source to an observer. In this case, the wave travels the fixed distance across which the source and the observer are located. But there are cases when either of the two is moving, that is, the source is moving relative to the observer, or vice versa. It is in these scenarios that the Doppler effect comes into picture. In this section, we will discuss Doppler effect of light waves.

### Doppler Effect:

Let us say that there is no medium between a source and an observer. The light source is moving away from the observer. Then the successive waves have to travel a larger distance as compared to the previous waves to reach the observer. Consequently, the time taken by the wave to reach the observer also increases. This results in the change in frequency of the wave.

The change in frequency or wavelength of light wave due to the relative motion between the light source and the observer is called the Doppler’s effect.

The Doppler effect is also observed in the sound waves. But since the light waves travel in vacuum as well and the speed of light is very large, the classic Doppler effect cannot be applied here.

### Blue Shift and Red Shift:

• When the light source moves away from the observer, the frequency received by the observer will be less than the frequency transmitted by the source. This causes a shift towards the red end of the visible light spectrum. Astronomers call it as the red shift.
• When the light source moves towards the observer, the frequency received by the observer will be greater than the frequency transmitted by the source. This causes a shift towards the high-frequency end of the visible light spectrum. Astronomers call it as the blue shift.

Red Shift and Blue Shift

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#### Practise This Question

What is the angle between the electric field vectors at points A and B because of an infinitely long line charge as shown?