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Different atoms emit different colors
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Spectoscopy is the science of identifying atoms by the light they emit. Each kind of atom emits light of a particular color. By examining the light emitted by a large number of atoms we can determine what kind of atoms they are, even if we can't observe the atoms directly. In fact, using spectroscopy helium was first discovered in the Sun ("helios" means "sun" in Greek) before it was ever observed on Earth!
The color of light is determined by its frequency. The lowest frequency light we can see looks red, the highest frequency looks violet. Radiation of frequencies lower than red is called infrared. We can't see infrared radiation but we can feel it as heat. Radiation of frequencies higher than violet is called ultraviolet. We can't see that radiation either, but it's what makes our skin tan when we go out in the sun.
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Photons are particles of light. Each photon carries a definite amount of energy. The higher the frequency of the light, the more energy the photon carries. So blue photons carry more energy than yellow ones, for instance.
Atoms can exist in more than one energy state. When they make a transition from one energy state to another they either emit or absorb a photon.If the atom goes from a higher energy state to a lower one, it emits a photon; if it goes from a lower energy state to a higher one, it absorbs a photon. In both cases the energy of the photon is exactly equal to the change in the energy of the atom. This means that each kind of atom emits and absorbs only photons of particular energies, frequencies, and colors.
We will see how this works on the next page.
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