All the light you see originates in atoms. They absorb energy from one source and emit it as different colors of light. When they receive energy, electrons surrounding the atom's nucleus move a little further out. The electrons immediately jump back, and when they do, they emit bundles of light energy, some of which you can see, and some of which takes the form of infrared light, ultraviolet or X-rays.
Electrons in an atom occupy paths, called orbitals, surrounding the nucleus. Strict rules govern how many electrons can fit in an orbital. If an atom has many electrons, their orbitals form shells, with the lowest-energy electrons in the inner shells and higher-energy electrons in the outer shells. Normally, an atom's shells are completely packed with no free space in the middle. If an atom absorbs energy, however, some of the outer electrons jump to higher levels. When it releases energy as light, the electrons return to their former levels.
Almost any energy source can make electrons jump to higher levels. Electricity in a light bulb or light-emitting diode (LED) makes electrons shift levels. Heat from a fire, or chemical and nuclear reactions also make electrons jump. If the atom receives enough energy, it loses electrons altogether, temporarily becoming an ion.
Conservation of Energy
After electrons move up energy levels, they release the energy, like a stretched spring bouncing back. A law of physics called the conservation of energy states that energy moves from place to place and changes to different forms but always has the same total amount. In effect, the energy the atom absorbs changes to light when the electrons fall back to their normal orbitals. Several electrons can emit light at the same time, so energy comes out in different amounts than how it arrived.
The orbitals that electrons occupy represent different levels of energy. The levels have fixed, discrete locations like the steps on a staircase, though unlike a staircase, differences between steps are not all the same. An electron dropping a single level emits light of a certain energy and color. An electron dropping a few levels emits light having greater energy and of a different color. When large numbers of electrons in a material emit light with different energies, it produces light of many different colors mixed together. This is white light.
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