Posted in Physics Revision

The Doppler Effect and Red + Blue Shift

The Doppler Effect

Definition: a change in the wavelength and frequency of a wave caused by relative movement of the wave source and the observer.

This occurs in all waves → sound, electromagnetic, etc.

When the source of a wave is moving relative to the obeserver it means that the waves produced are compressed, as shown in the diagram, if the source is moving towards you.


This means that the wavelength has decreased.

As the speed of the wave remains the same, the frequency has to increase.

We know this by looking at the equation for wave speed:

v = f x λ

Remember, this only occurs if the source is moving at a different speed to the observer. If they are moving at the same speed, the effect doesn’t occur.

So, because the frequency appears to have increased, the sound is higher pitched.

If the source was moving away, the wavelength would be longer, so frequency would be smaller, meaning the sound would be lower pitched.

This effect is exhibited best when a siren drives past you; the pitch appears to change depending on how far away from you the siren is, and whether is is coming towards you or moving away.

Image result for doppler effect diagram

Red and Blue Shift

Red Shift Definition: The increase in wavelength and so decrease in frequency of electromagnetic radiation from distant, receding galaxies due to the Doppler effect and the expansion of the Universe.

So, essentially, this is the Doppler effect, but with light or other electromagnetic radiation. In the same way sound appears more high pitched when the source moves towards us, light appears more “blue” when it is moving towards us, and more “red” when it is moving away.

Red Shift as Evidence of the Big Bang

The big bang theory states that a huge explosion occured about 13.7 billion years ago that released so much energy that energy became matter, and the universe was born. It also states that the universe continues to expand to this day.

We can use red shift as evidence for this. This is because when electromagnetic waves move away from us, they are red shifted as their wavelength appears longer. The light we see from stars across the universe is red shifted; this means they are moving away from us. Since they are moving away, it indicates that the universe is expanding, as if it was staying the same, the light would exhibit no red or blue shift.

For example, if 3 objects were floating in a balloon, and that balloon was inflated more, then the volume of the balloon would expand, and the objects would move further apart.

Also, the more red shifted a star is, the further away from us it is. So we can use the amount of red shift a star exhibits to find out how far away it is from us.


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