Phosphorescence

What is the wavelength of light being given off of glowing objects? Is it the same wavelength as the origional light the phosphors were energized with? Is it possible to take the energy out of the glowing object (lessen the glow of the object) by shining a different type of light (for instance infrared) that has a different wavelength, therefore bumping into and messing up the wavelength of the light emitting from the glowing object?
- Lindsey (age 15)
San Diego, CA, USA

The phosphorescent emission is generally at lower frequency, longer wavelength than the original light. Typically, the original light is absorbed putting a phosphor in a high-energy state. It then trickles down, emitting some small amount of energy, to a state which emits light very slowly. (This state is a triplet spin state, for those familiar with that term.) From that state, light is emitted over a longer period of time. Since the long-lived state has lower energy than the first high-energy state, the amount of energy given off when falling back to the lowest energy (ground) state is not as big as the amount absorbed initial. That means that the outgoing unit of light (photon) has less energy than the incoming one. There’s a simple relation between energy and frequency (E=hf, where h is Planck’s constant.) so the outgoing frequency is lower.

Shining infrared light on won’t directly ’mess up’ the outgoing light, but it could jostle the phosphor into a state from which it will emit more easily (and at a higher frequency) than from the triplet state. However, at room temperature, the lower frequencies of infrared light are always present, so for a good phosphor one would expect that this effect must be small.

Mike W.

(published on 10/28/2007)