![]() Now we've got an example where a space ship is moving towards a star and it speeds up, alright? So it's going towards, that means it's squishing the wavelength, it's making it smaller. So when things have a larger wavelength, we say that they're red shifted. Now, when we think about visible light, we think 400 nanometers to 700 nanometers. ![]() So the star's moving away from us and so that means that the wavelength is going to be bigger. It works basically with any type of wave. Now, what's the wave that we're perceiving from the star? The light wave. So what about number 2? A star is moving away from the earth, alright? Now it's moving away. So that means that it's going to squish the wave front even more so that means that the frequency will get higher. Now, if it's honking its horn while it speeds up, what will happen to the frequency that you're perceiving? Well, it's moving towards you but it's speeding up. A car's moving towards you and it speeds up. There are of course formulas for that but most introductory Physics courses don't require that you actually calculate that stuff. Now I'm not going to really talk about this quantitatively, like how could you calculate the frequency. Let's do some example problems involving this. That means the wavelength is longer, the frequency is smaller. ![]() It's moving away, it stretches out the wave. Wavelength is shorter, that means the frequency is bigger. It's a standard easy, very very very simple thing to understand when you look at how it works. So, as you can see the car starts, here it is at rest and now he's going to move and look at that, squished in front, stretched in back. So the wavelength is larger and the frequency is smaller. Alright? On the other hand, when the two objects are moving away from each other, then the wave is going to be stretched. So that means that the wavelength is smaller and the frequency is larger than it would be if there wasn't any relative motion. Alright? So that means that when the source and the observer are moving towards each other, the wave is squished. The wave will be squished in front and it will be stretched in back. So that means, that the wave that it's creating is going to be perceived differently by me than it would be if the ambulance was just sitting there parked, playing its siren. So I could have for example an ambulance with a siren and it's moving towards me. What is the Doppler effect? The Doppler effect is something that happens when you've got either a source of a wave or an observer of a wave or both in relative motion. This article concerns itself only with Doppler shifts.So let's talk about the Doppler effect. They describe the total difference in observed frequencies and possess the required Lorentz symmetry.Īstronomers know of three sources of redshift/ blueshift: Doppler shifts gravitational redshifts (due to light exiting a gravitational field) and cosmological expansion (where space itself stretches). The relativistic Doppler effect is different from the non-relativistic Doppler effect as the equations include the time dilation effect of special relativity and do not involve the medium of propagation as a reference point. The relativistic Doppler effect is the change in frequency, wavelength and amplitude of light, caused by the relative motion of the source and the observer (as in the classical Doppler effect), when taking into account effects described by the special theory of relativity. The frequency is higher for observers on the right, and lower for observers on the left. A source of light waves moving to the right, relative to observers, with velocity 0.7 c.
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