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Red Shift: Definition, Theory & Effect

Instructor: Amanda Robb
This lesson is about red shift, a phenomenon of waves. We'll go over the fundamentals of red shift, waves and the Doppler effect. Then we will talk about why red shift is important and how scientists are using it to answer questions about the universe..

What is a Wave?

Red shift is a phenomenon that has to do with waves. So before we get into red shift, we have to make sure we are clear about the properties of waves. Waves aren't just how you get someone's attention across the room. In fact, in physics, we have a fancy definition for a wave: It is a disturbance in a medium that transmits energy from one point to another. The medium is what the wave travels through. A sound wave is a type of wave that causes collision of air molecules, which is the medium (we interpret it as sound). A light wave is another type of wave that travels through space or air on Earth.

Waves have a specific shape. There are peaks called crests, and the wavelength is the distance between crests.

Diagram showing how the wavelength of a wave is measured
wave diagram

The Doppler Effect

Maybe you have heard of the Doppler effect, but aren't sure exactly what it is. It's more common than you might think, and we all experience the Doppler effect in everyday life! A classic example is how the siren of an ambulance changes pitch as it moves past. The reason this happens is because of the Doppler effect. In the Doppler effect, as a wave source moves away from us, the wave spreads out and wavelength increases. If the source of the wave is coming closer to us, the wave compresses, and the wavelength gets shorter.

The pitch of the siren is related to the wavelength. As the ambulance comes closer, the waves compress, the wavelength gets smaller, and we hear the noise as a higher pitch. Sound waves, such as the siren, and light waves are both affected by the Doppler effect. The image below shows an object moving toward and then away from a source (the red dot) and the wavelength (the blue lines) compressing as it gets closer to the source then spreading out as it moves away.

The Doppler effect causes the wavelength to compress or spread out as the source moves closer or further away.
doppler effect

Red Shift

To understand red shift, we also have to understand the spectrum of light. Light with long wavelengths are on the red end of the visible light spectrum, or light we can see. Violet light has the shortest wavelength of visible light, as shown below.

Red light has the longest wavelength in the visible spectrum.
visible light spectrum

When light is red shifted, it means that the wavelength is increasing due to movement of the wave source away from the detector, or person seeing the wave. In red shift, the light appears more red, because the wavelength is getting longer. The Doppler effect says that wavelength increases as objects move away from us, so light that is red shifted must have a light source moving away from us. Below is a diagram of a star moving away from us, which causes the wavelength of light to increase, creating more red light.

When the source is moving farther away, the wavelength increases and the light appears red shifted.
red shift

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