An overview of absorption of electromagnetic radiation. This example shows the general principle using visible light as a specific example. A white light source —emitting light of multiple wavelengths —is focused on a sample (the pairs of complementary colors are indicated by the yellow dotted lines). Upon striking the sample, photons that match the energy gap of the molecules present (green light in this example) are absorbed, exciting the molecules. Other photons are scattered (not shown here) or transmitted unaffected; if the radiation is in the visible region (400–700 nm), the transmitted light appears as the complementary color (here red). By recording the attenuation of light for various wavelengths, an absorption spectrum can be obtained.
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