luminescence
/ loo-min-ESS-ense /
A hot stove glows red because it is hot, but a firefly, a glow-in-the-dark star, and a highlighter pen under UV light all glow while staying cool. That cold glow is luminescence: a material absorbs energy and then gives it back out as visible light, rather than as heat. It is light produced by an electronic trick instead of by temperature, which is why it is sometimes called cold light.
The mechanism is a two-step electron journey. First the material absorbs energy, from ultraviolet light, an electric current, or a chemical reaction, and an electron is kicked up across the energy gap to a higher level. There it is unstable, and when it falls back down it releases the difference as a photon of visible light. Because the electron typically loses a little energy to vibrations before falling, the emitted light is usually of longer wavelength (lower energy) than what was absorbed, which is why an ultraviolet lamp makes materials glow in visible colors. The timing splits luminescence into two types: in fluorescence the electron falls back almost instantly (within nanoseconds), so the glow stops the moment the source is switched off, while in phosphorescence the electron gets stuck in a metastable trap and dribbles back out over seconds, minutes, or hours, which is the lingering afterglow of glow-in-the-dark toys.
Luminescence is engineered light. Fluorescent lamps coat the tube with a phosphor that converts the tube's ultraviolet glow into white visible light; white LEDs do the same trick, wrapping a blue chip in a yellow phosphor. Old cathode-ray televisions painted the screen with red, green, and blue phosphors struck by an electron beam, and modern displays, glow paints, security inks, and biological stains all run on the same physics. The choice of luminescent material sets the emitted color, because the size of the energy gap it drops across fixes the photon's energy. A useful distinction: fluorescence needs the excitation kept on and dies instantly without it, whereas phosphorescence keeps glowing after the lights go out, and both are quite different from the hot glow of an incandescent bulb.
A white LED is a blue chip coated with a yellow phosphor: the phosphor absorbs some blue light and re-emits yellow, and blue plus yellow reaching your eye looks white.
An electron absorbs energy, then re-emits it as a visible photon; fluorescence is instant, phosphorescence lingers.
Fluorescence stops the instant the excitation is removed, while phosphorescence keeps glowing for a while because the electron is briefly trapped; both are cold light and differ from the temperature-driven glow of a hot filament.