My understanding is that when an object moves away from you it increases the wavelength of the light presenting with a colour that is on the red side of the colourscale. Scientists also are able to determine what elements are present on planets based on the light it emits. How can they tell the difference?
I’m sorry for possibly using the incorrect terminology! Thanks in advance folks!
Edited the post because previously I had suggested that shorter wavelengths tended to the red side when in fact longer wavelengths tended to the red side of the colour spectrum.
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Red shift tends to be noticeable from the light from *galaxies*, not distant planets. Even the furthest planets we can see are within our own galaxy, so moving with us. It is distant galaxies that are moving away rapidly.
But yes, it is a fun question. The answer is that different elements have very specific “signature” patterns of light. For example, something with hydrogen in it, that is glowing, [looks like this](https://en.wikipedia.org/wiki/File:Emission_spectrum-H.svg). You get very narrow spikes of light in the purple, blue, light blue and red part of the spectrum, and nothing anywhere else. Going beyond just visible light [you get something a bit more complicated](https://en.wikipedia.org/wiki/File:Hydrogen_spectrum.svg), but the same sort of thing – very specific wavelengths of light.
You also get the same sort of thing in reverse, with absorption spectra; when light travels through materials specific wavelengths of light will be absorbed (e.g. [this for hydrogen](https://en.wikipedia.org/wiki/File:Spectral-lines-absorption.svg)).
So what astronomers do is look at the light coming from or through something, look for the gaps (if the light is passing through the thing they are interested in) and spikes (if the light is being emitted by the thing), and do some complicated analysis to figure out what combination of elements could give those patterns.
For red shift, you have some idea of what to expect (you know that galaxies are mostly hydrogen) so you look the distinctive hydrogen lines, but find them in the wrong place. How far off they are tells you how much the light has been red-shifted. For example, going back to our [hydrogen emission spectrum](https://en.wikipedia.org/wiki/File:Emission_spectrum-H.svg), the gaps between the bright spots have very specific widths (a narrow one, a wider one and a huge one – and this pattern is repeated across the whole spectrum). So if you get a bunch of light that has come from a galaxy, and you find there are spikes and wavelengths with the same gaps but in a different part of the spectrum (say the first spike is in blue, then the close one is in light blue, the next in green, and the one after is infrared) that’s a good hint that you’re seeing light from hydrogen that has been shifted a bit.
To use a (bad) analogy from your comment history, think about different formations used by rugby players, for different set pieces; a line-out looks largely the same wherever in the pitch it is happening. Someone who knows the game would be able to tell a line-out from a scrum (or equivalent – rugby isn’t my speciality). Looking at the positioning of the players an observer could tell whether they’re about to do a line-out or a scrum (different elements), and whether it is taking place at one end of the pitch or the other (how red-shifted).
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