I just can’t compute how this is possible. How can a telescope detect a gas, which isn’t even visible to the naked eye, on a planet that is an incomprehensible distance away.
[Source](https://www.bbc.co.uk/news/science-environment-66786611)
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Light from a broad spectrum source produces a full rainbow spectrum, but as this light passes through any gas, that gas absorbs certain frequencies of light, resulting in slight dark patches on the spectrum. See this video explanation:
# VT.Physics | [Emission and Absorption Line Spectra – A Level Physics](https://www.youtube.com/watch?v=jsNkcCyrxvU)
For example, when people say that CO2 is a greenhouse gas, and that it traps heat, what this means is that infrared light on the light spectrum gets absorbed by CO2, and gets re-radiated. So heat from the earth, after it has been warmed by the sun, which would normally be re-radiated into outer space as infrared, gets absorbed by the CO2 in our atmosphere, and gets re-radiated in all directions as a sort of infrared glow in the sky, kinda like how our sky is blue, where a mix of light, primarily blue light, gets absorbed and re-radiated in all directions by our atmosphere.
This is how we can optically assess the composition of our atmosphere. Every type of gas has its own fingerprint of a line spectrum.
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