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All elements, when hit by light, reflect it at varying frequencies and by looking at the frequency of the reflected light it is possible to find out the elemental composition of an object, even from very far away.

But let’s say there was a previously undiscovered element (it’s very likely there are some out there). What would it look like on the spectral analysis? Would it necessarily have a different signature than all the others discovered elements? And how many things would you be able to tell about the element just by looking at its light reflection spectrum?

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    $\begingroup$ Emission/absorption spectroscopy (which I assume is what you are talking about) is not based on reflection but, as the name suggests, on emission or absorption. And you have answered your own question. What is your worldbuilding problem? $\endgroup$
    – L.Dutch
    Jul 30 at 11:11
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    $\begingroup$ (1) "Looking at the frequency of the reflected light it is possible to find out the elemental composition of an object, even from very far away": Looking at the spectrum of the reflected light all you can learn is the composition of the light source. You cannot tell is the reflector is made of glass, bronze, silver or aluminium. (2) You may want to research why helium is named helium. (It was first discovered in the spectrum of solar light. Only later was it also found on Earth.) $\endgroup$
    – AlexP
    Jul 30 at 13:08

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You can sometimes tell quite a lot from remote sensing and looking at the reflection from an object. It can be illuminated by your own laser like in LiDAR or reflected light from the sun. However, you are not usually looking at a single element or compound unless it is in a laboratory with a prepared sample. In some cases you can use a technique called Raman spectroscopy and if you hit the sample with a laser a very small percentage of the photons that bounce back will have a signal that is related to the molecular bond vibrations of the sample and that fingerprint will tell you what the substance is.

On terms of unknown elements, science is pretty sure that we have found them all or can predict them. The undiscovered elements we think are there but have not been shown experimentally are very heavy, very radioactive and have very short half lives. There is a theory of an “Island of stability” where if elements could be synthesized to have a larger number of neutrons they would have longer half-lives.

island of stability

Like all the other elements if you wanted to excite their electrons and look at the optical emission you should see a set of atomic lines that should be unique to that element.

If they were an alloy or a compound looking at the reflections it might be hard to determine that it was a new element from far away, but as you got closer and if you zapped it with a laser or vaporized into a plasma by some other method, to the eye it might look like a normal plasma but with a spectrometer it would have a unique signature.

If it was a large object made of a pure material, you might be able tell it was unusual by reflection and maybe if you probed it across the whole EM spectrum tell that it was something new. But most things except metals are compounds so that would make it harder to tell.

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  • $\begingroup$ Very thorough answer. Thank you $\endgroup$ Jul 30 at 13:53
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Yes. This is one way to look for unusual types of matter.

DETECTION OF AN UNIDENTIFIED EMISSION LINE IN THE STACKED X-RAY SPECTRUM OF GALAXY CLUSTERS The Astrophysical Journal, 789:13 (23pp), 2014 July 1 Bulbul et al.

As for dark matter, 80 yr from its discovery by (Zwicky 1933, 1937), its nature is still unknown (though now we do know for sure it exists, from X-ray and gravitational-lensing observations of the Bullet Cluster; Clowe et al. 2006, and we know accurately its cosmological abundance, e.g., Hinshaw et al. 2013). Among the various plausible dark matter candidates, one that has motivated our present work is the hypothetical sterile neutrino that is included in some extensions to the standard model of particle physics...

In this paper, we undertake a fishing expedition that combines the spectra of many bright clusters from the XMM-Newton archive in order to search for any kind of faint, unidentified X-ray emission lines—be they thermal lines from previously undetected rare elements in the ICM ( intracluster/intergalactic medium ) or the elusive sterile neutrino decay line.

I really like "fishing expediton" which so often is used as a pejorative insult when applied to science. Not here. Know thyself!

The premise is exactly as you lay out. The spectral lines associated with a type of matter characterize that type of matter. You could use the presence of unaccounted-for spectral lines to deduce the presence of previously unknown matter. Chem stack has a fine overview.

https://chemistry.stackexchange.com/questions/126917/how-can-every-atom-have-unique-spectral-lines

This overview refers to the Sharp series and other series which characterizes emission spectra according to the electron shells of the emitting atom or ion. The spectrum of an emitter is not random but arises from its structure. Heady stuff. But I take away that one could derive the nature of an element emitter from the emission.

Lines will differ for different ions of an element and different isotopes. Lines can be frequency shifted according to the speed (relative to the viewer) of the emitting atom - an example is the accretion discs of black holes, where the known spectrum of iron is shifted both coming and going and that shift can be used to calculate the rotational speed of the disc.

Whether nonbaryonic matter has associated brightline spectra is not clear. Dark matter must not do it like baryonic matter or it would be visible! "Undiscovered elements" is questionable given how predictable our familiar elements seem to be. That there could be undiscovered forms of matter not on the periodic table is not questionable at all and a lot of money is spent on searching for these particles.

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