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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ELI5: Every element absorbs (takes in) and emits (like a bulb) light at VERY specific wavelengths. Hydrogen, for example, emits/absorbs at 656 nanometers, 486 nanometers, 434 nanometers, and 410 nanometers (in the visible light range.) So we know, since hydrogen is… pretty much everywhere, that we should ALWAYS see those 4 lines exactly that far apart from each other. There are similar numbers for EVERY single element.
So what we do is we look for lines that are EXACTLY that far apart from each other. ALL of them will be redshifted (toward higher wavelengths), but the distance between them will remain the same. And that’s the important part.
They can tell what elements are present in a distant planet simply by limiting the view of the telescope (with a light detector attached to it, called a “spectrometer”) to that planet. If they can ONLY see light from that planet, then ALL of the wavelengths detected will be from the elements on that planet. (It’s worth noting that you will ALWAYS see the light from the star near that planet. And it’ll look like something called “blackbody radiation”. The emissions from the elements will be on top of this curve.) Look at this image of the sun, you can see the “dips” in the spectrum. These are due to specific molecules. The “dips” due to atoms (not molecules) are far, FAR smaller, you’d have to zoom in a ton to see them.
https://www.e-education.psu.edu/meteo300/sites/www.e-education.psu.edu.meteo300/files/images/lesson6/Solar_spectrum_en.svg.png
These absorptions and emissions are called the “atomic spectral series” or just the “atomic spectra” of any particular element.
Every element has it’s own “series” of these emissions. And that series never changes, it just shifts to higher wavelengths. That “series” is related to how far apart the energy levels of the atoms are (hence why they never change, as the energy levels are solutions to a particular equation, and math never changes), but this is slightly beyond a ELI5 explanation.
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