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.
In: 17
(To condense/simplify the answer from grumblingduke)
All atoms absorb and emit light slightly differently which means that different atoms end up having different [spectral line](https://en.wikipedia.org/wiki/Spectral_line) patterns. These patterns essentially act like unique fingerprints.
When relativistic speeds get involved, the bright lines in these patterns might become more stretched out (redshift) or scrunched up (blueshift) but the proportional distances between lines keeps the fingerprint recognizable.
Furthermore, because multiple different elements from the same star/galaxy/etc. will be moving at the same relativistic speed that means that comparing the stretch/scrunch of multiple different fingerprints means we can double-check our work. Either they’re all stretched/scrunched by the same amount (because they’re going the same speed) or somebody made a mistake and we have to reevaluate what fingerprint(s) may have been misidentified.
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