The Doppler Effect

What is the Doppler Effect?

The Doppler effect effects moving objects, there appearance and the sound (if the object makes any). Firstly, the sound. In the example to the right we can see a police car with its sirens on. As the car moves forwards, the waves at the front of the car decrease in the distance between them as they are being compressed, this is known as having a smaller wavelength . This effects the frequency, making it higher and gives the sound a higher pitch. Contrast to this, on the other side the waves are actually spread out, or known as having a long/larger wavelength, this in return means that frequency will bee lower and the pitch lower.

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But how about the appearance? Well, it works in the same way as sound however pitch is regards to colour and frequency is wavelength. This might not make sense if you haven't covered waves but, waves with small wavelengths on the colour spectrum are further to the left or the blue side, this is known as a blue shift. The same goes for larger/wider wavelengths however it is the other side of the spectrum, red. This is known as a red shift.

How can we tell that the universe is still expanding from the Doppler effect?

We can tell this due to upon observing far away galaxy's, they have a slight red tint - red shift. This tells us that they are moving away from us therefore the universe is expanding. To find this out we use an absorption spectrum. Here we pass the light from the galaxy's through the paper. Next we compare it with the absorption lines from those created in a lab (from our sun). below you can see that the middle one shows the light at rest, once the object begins to move we can start to see clear shifts due to the difference in wavelengths effected by the objects movement.

To calculate the Doppler Shift we first choose a reference line from the resting spectrum and compare it with the shifted spectrum line. There we measure the change in the wavelength and we can calculate the speed. We use the below formal to do this:

Change in wavelength = Velocity of a galaxy

Reference wavelength Speed of light