When an atom is excited from one energy state to a higher stage by the absorption of a photon, it may return to the lower level in a series of two (or more) jumps if an energy level exists in between. The photons emitted will, consequently, have lower energy and frequency than the absorbed photon. This phenomenon is called fluorescence; common fluorescent rocks and paints can emit visible light after absorbing UV light.
Fluorescence is responsible for the appearance of objects under the so-called "black light," which is a source of ultraviolet radiation. Photons in the ultraviolet region, invisible to the human eye, have higher energies than those in the visible region, and hence, if an ultraviolet photon is absorbed by an atom, the outer electron (which is responsible for the visible transitions) can be excited to high levels. These electrons make transitions back to their ground state accompanied by the emission of photons in the visible region.
Objects seen in ultraviolet light often show colors in the blue or violet end of the spectrum that are not present when the objects are viewed in sunlight. We can understand this effect by considering the composition of sunlight and the optical excited states of a typical atom. The intensity of sunlight is concentrated in the center of the visible spectrum, in the yellow region; very little intensity is present in the red or blue ends of the visible spectrum. The "yellow" photons have enough energy to excite an atom up to its lower levels, but not enough to reach the higher levels. However, the higher-energy uV photons do have sufficient energy to reach the higher level, so the light emitted by the atom has a stronger blue component when that atom is excited by ultraviolet light than when excited by sunlight.
The wavelength at which fluorescence will occur depends on the energy levels of the particular atoms. Because the frequencies are different for different substances, and because many substances fluoresce readily, fluorescence is a powerful tool for identification of compounds.