When an atom gains energy from a collision with another atom, an electron, or a photon of light, the atom is said to become “excited.” Upon excitation the electrons in the atom jump to higher energy levels. The “excited” electrons quickly drop down to lower energy levels, resulting in the emission of photons with energies equal to the differences between the energy levels (Fig. 4.9). Higher energy levels correspond to greater average distances between the electrons and the nucleus. Most important, the energy of an emitted photon is directly proportional to its frequency and inversely
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