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Each atom or molecules has an energy level structure which determines how electrons behave within the atom or the molecule.
Light is absorbed by a
material when the frequency of the light
is in resonance with the
"natural" frequency of that material.
Since there is an energy transfer, the atom will move to an
upper energy state or
excited state.
Scientists identify materials of
unknown composition by
determining the absorption spectrum
of the material.
By "shining" light at different wavelength on the
material, they can determine at which wavelength(s) light is absorbed.
Since each atom and molecule has a "signature", they can determine which atoms are
present in the material.