This effect of physics observed by the physicians Doppler and Fizeau have led to major discoveries in the world of science, and especially in astronomy.

Have you ever asked yourself why, when an ambulance passed, the sound seems deeper when it has passed? Or what really is the sound barrier? These are basically applications of the Doppler-Fizeau effect.

Imagine a source of stationary waves-in this case sound. All waves disperse in circles. Here all those“circles” have the same center. Look at (a) from the picture:

Now imagine that this source is moving in a certain direction: You get something like (b):

This odd figure is caused by the moving source, and the place where a new sound wave is created is different to where an old one was. As you can see in figure B, the waves are denser in the direction of the movement than in the direction opposite of the movement. This creates a change in frequency (measured in Hertz: Hz.)
Therefore the frequency in the direction of the movement
is higher than in the direction opposite of the movement, where it it lower. The sound you hear depend on there frequency, the lower the frequency the deeper the sound. You got it for the ambulance!

What is the sound barrier then?
Imagine the source going faster than its own waves. You get a cone-like shape. This creates a shock wave known as a “bang” noise.

What were the implications in astronomy?
Hubble, the American astrophysician, discovered, when looking at spectrograhies from other galaxies that their light shifted to red. He deduced that the frequency was getting lower due to the Doppler-Fizeau effect, and that therefore the galaxies were all moving away from each other at a speed proportional to their distance. This phenomenon is known as the expansion of the universe and is a pillar of the Big Bang theory.