What Is Black Light?

Black light bulbs emit light only in the ultraviolet spectrum. Ultraviolet rays are invisible to the naked eye, thus the descriptor "black."

Black Light and Phosphors

Any material that appears to glow under black light has phosphors embedded in it. Phosphors absorb radiation and emit visible light in response--materials that display this property are said to be phosphorescent. Another term for this process is fluorescence.

Phosphors In Your Laundry

Modern laundry detergent has phosphors deliberately added to it to create a "whiter than white" effect. When ultraviolet rays strike a white shirt in, say, natural sunlight, the shirt will seem extra-bright. The shirt isn't any cleaner; it's just glowing. Many laundry additives, such as bluing compounds, also contain phosphors for this same reason.

Phosphor-Free Alternatives

Not all laundry detergents contain phosphors, especially the new generation of all-natural, eco-friendly products. It can be hard to tell which ones do and which ones don't, however, since by law detergents are not required to list their ingredients. Generally speaking, any detergent that lists a "brightener" or brightening agent will contain phosphors. So conversely, if a detergent says it contains "no brighteners," it means no phosphors--and no glow.

Fun Fact

You can tell whether a detergent contains phosphors by shining a black light directly on the detergent--if it does, it will glow.

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How Stuff Works: How Black Light Works