since different gases give different color when burning, how can we know that the red colour we obtained is not just the actual color of gases burning, but rather a red shift?
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If you pass the light from an object through a prism and inspect it closely, there will not be a smooth, continuous rainbow. There will be gaps in the colors.
These gaps are caused by the presence of certain atoms and molecules, either in the thing emitting the light itself, or in obstacles between us and the thing of interest.
Each atom or molecule takes a very specific “bite” out of the rainbow. Like a unique signature. We know what these signatures look like and where on the rainbow they are supposed to be.
If you notice a signature of an atom or molecule that has the right shape, but isn’t bitten out of the correct color, that’s how you know it’s been shifted, and you can “unshift” all the data back to where it should be to derive the original color. If it’s exactly where you expect it to be, then you’re already seeing the object in true color and no shift is necessary.
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