How To

# How the Doppler Shift Works

Waves of light and sound both move like, well, waves. Different waves have different wavelengths depending on a few factors, and the lengths of these waves can help us identify what they’re coming from and at what speed. This is the essence of the Doppler Effect, also called the Doppler Shift. Here’s how it works and why it’s important.

1. An object emits waves of sound or light.
For both sound and light waves to occur, they have to be coming from something. For example, a train whistle sends out waves of sound, and a distant star or galaxy sends waves of light (both visible and invisible).

2. Depending on where the observer is, the waves change in frequency.
Let’s take the famous train example to start. Imagine you’re standing on the platform of a train station. When the train is moving towards you, the sound waves are bunched up and shorter.. When the train moves away from you, the sound waves stretch out. This is all because the waves take different times to reach you. The sound itself is the same when it leaves the train whistle, but you as the observer will experience different wavelengths because it has to travel a further or shorter distance. The same thing happens with light, but with different results.

3. Sound waves change in pitch, while light waves change color.
When the train is moving towards you and the sound waves are shorter, the pitch of the sound is higher. When the train is moving away and the waves are longer, the pitch is lower. This is the classic Doppler Effect example because it’s easy to imagine. When you’re waiting for a train to pass, the sound gets higher and higher until the train is passing you, when the sound fades lower and lower.

With light, the differing wavelengths result in different colors, not sounds. This is what we call redshift and blueshift. If a light-emitting object is moving towards us, the shorter wavelengths result in a blueshift (the color is shifted towards blue). If a light-emitting object is moving away from us, the color is shifted towards the red (redshift). This is a fundamental concept in astronomy that led us to the conclusion that the vast majority of galaxies in the observable universe are rapidly moving away from us.

#### rod

Good report. I think it is important to point out that the doppler shift is not used to explain galaxy redshifts in cosmology. If I recall correctly, these are cosmological redshifts caused by light traveling through 3-D space expanding, not the speed of recession. The physics come from Einstein General Relativity and principle of equivalence as I recall.

#### twr

I personally do not like the light / sound comparison. Because the sound waves has a medium to be transported but light has not! And sound become slower when the train moves away. But light is as fast as it is whether static or moving.

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