OBJECTIVE: To see how atoms emit light.
Notice the brilliant colors that result. The solution containing Li+ gives a beautiful, deep-red color, while the Cu2+ solution burns green. Notice that the Na+ solution burns with a yellow–orange color, a color that should look familiar to you from the lights used in many parking lots. The color of these “sodium vapor lights” arises from the same source (the sodium atom) as the color of the burning solution containing Na+ ions.
As we will see in more detail in the next section, the colors of these flames result from atoms in these solutions releasing energy by emitting visible light of specific wavelengths (that is, specific colors). The heat from the flame causes the atoms to absorb energy—we say that the atoms become excited. Some of this excess energy is then released in the form of light. The atom moves to a lower energy state as it emits a photon of light.
Lithium emits red light because its energy change corre- sponds to photons of red light (see Figure 11.8). Copper emits green light because it undergoes a different energy change than lithium; the energy change for copper corresponds to the energy of a photon of green light. Likewise, the energy change for sodium corresponds to a photon with a yellow–orange color.
When atoms receive energy from some source—they become excited—they can release this energy by emitting light. The emitted energy is carried away by a photon. Thus the energy of the photon corresponds exactly to the energy change experienced by the emitting atom. High-energy photons correspond to short- wavelength light and low-energy photons correspond to long-wavelength light. The photons of red light therefore carry less energy than the photons of blue light because red light has a longer wavelength than blue light does.