Home Borrisoleigh The Doppler Effect and Red Shift

The Doppler Effect and Red Shift


By Aaron Reid, 5th year, St. Joseph’s College, Borrisoleigh

One of the topics I have studied this year, that interested me was sound. One of the principles we studied was the Doppler Effect.

What is the Doppler Effect and why does it occur daily in our lives without many of us actually understanding what it is. Most of us have only ever heard it from Sheldon Cooper but what is it ?


It was firstly proposed by Christian Doppler, in 1842, who said the Doppler Effect is the apparent change in frequency due to the relative motion of either the observer or the source.

Now this might sound complicated but in simpler terms, why does the sound of an ambulance or any siren that is passing change as it approaches us and again as it speeds away?

This is because the engine noise appears to have a different frequency as can be seen below.

You may be wondering how the Doppler Effect can be used in physics ?

One advancement made using the Doppler Effect is learning that the universe expands.


The stars we see in the sky shine with a bright yellow hue. However, scientists observing these stars with telescopes, have noticed the colours of the stars are changing ever so slightly.

They are moving towards the red end of the spectrum.

As stars move away we see them as a red colour, and therefore it iis called the Red Shift. Therefore it can be concluded, that these stars are moving away, so the universe must be expanding. Yes, the universe is getting bigger!

The doppler effect is also used in Radar to measure the velocity of objects detected. The gardai use this principle with their speed guns.


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