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Spectroscopy is an analysis of light (or other EM wavelengths) that is often used by scientists to examine what an object is made from or contains.

Apologies if this is a stupid question, but this sounds effectively like saying “this thing is absorbing this wavelength or colour, therefore it is this substance”.

I’m confused as to how this works in practice. As a familliar example: light bouncing off leaves in summer can be a completely different colour to leaves in winter. Water when frozen looks to be a different colour to when it is liquid. Copper is shiny brownish unless it gets a bit rusty, then it’s bright green, etc etc.

Is it possible to explain how we can say with any amount of certainty that a given set of dips in EM wavelengths translates into identification of substances?

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    $\begingroup$ This is a great question! I've adjusted the wording a bit to better match the site, please feel free to edit further. I removed a few instances of "certainty" because generally scientists are a cautious bunch and don't really ever say anything like "X is certainly made of Y and Z" outright, and don't even believe in "likely made of" without a lot of corroborating evidence, discussion and peer review. $\endgroup$ – uhoh Sep 19 at 9:09
  • $\begingroup$ A loose analogy might be identification of individual people by biometrics e.g. fingerprints, DNA, retinal scans... There are a several of those and each provides (correctly or very disastrously sometimes incorrectly) a statistical match. My guess is that an answer here will be based in part on an explanation of likelihood or other statistical measures of the quality of spectral fitting, and catalogs of spectral features of known materials measured on Earth, or in those cases where they can not be reliably synthesized, theoretically predicted using standard, vetted simulation techniques. $\endgroup$ – uhoh Sep 19 at 9:14
  • $\begingroup$ I think you can ask a second question in parallel to this one if you are interested, something along the lines of "Notable examples of mis-assignments of spectral features that turned out to be reasonable but later were found to be wrong?" You may also be interested in how helium was first discovered. $\endgroup$ – uhoh Sep 19 at 9:17
  • $\begingroup$ In what kind of objects? $\endgroup$ – Rob Jeffries Sep 19 at 16:57
  • $\begingroup$ I was thinking of exoplanets, moons, planets etc $\endgroup$ – tomh Sep 20 at 16:13
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This is not exclusive to spectroscopy applied to astronomy but general.

Matter can interact with electromagnetic waves spanning a very wide range of frequency (energy). Also matter can emit electromagnetic radiation when in a kind of excited state.

Due to the internal mechanism of absorption/emission it happens that the spectral characteristics can be peculiar to the chemical nature and physical state of the matter under analysis.

This is basically what you are aware of. Your concern likely arises by over-focusing on colour, that per se in not the most powerful identification tool, or even by thinking that the particular state of the matter does not count, or changes are neglected.

Let me take your example of leaves. The fact that they colour undergoes changes does actually means that their composition changes. This is already an information rather than a problem.

Imagine that observing a planet one detect, say, carbon monoxide, it does not mean that its atmosphere won't change. Eventually different observations would prompt a planetologist to ask himself why, perhaps concluding that a kind of volcanic activity or degassing is ongoing , just to say.

A planet looking green then brown on a regular pattern would certainly suggest the presence of plants or anyway photosynthesis based on chlorophyll, providing that the green spectrum closely resembles that one we know, and does not come from narrow spectral features that would otherwise indicate, for instance, the presence of chlorine.

In other words, probing a sample, in astronomy as well as on a lab bench, leads to information about that sample in that moment. This is quite philosophical rather than a problem in spectroscopy or spectroscopy applied to astronomy. Moreover spectroscopy doesn't come alone and certainly requires considering the scenario.

But at its core is the existence of peculiar features, they can be single lines or more or less complicated spectra, and those are linked to the composition of the sample, elemental or molecular depending on the frequency window used.

I am not sure if this answer your question, but again this passage

"this thing is absorbing this wavelength or colour, therefore it is this substance”

is both correct and wrong. It must be taken with a grain of salt, or things must be analysed in depth.

What is certainly true is that there are specific lines typical of elements, or other spectral features typical of molecules. Explaining why it is so is way more complicated (for me) to be done in few lines here. But the reason resides on the discrete electronic structure of matter, as well as molecular shapes combined with limitations on which transitions can take places. Combining the two leads to a high specificity.

Unfortunately not the best example for astronomy, but consider that a given compound virtually has its own IR vibrational spectrum, different from that of any other compound!

You might want to read about spectroscopy in general, and have a look at Wikipedia's Astronomical spectroscopy; Chemical properties

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