What's commonly known as black light is actually ultraviolet (UV) light. Typical black light bulbs have a purple (i.e. violet) glow, since they emit both UV as well as some visible light.

Often used in clubs, bars and similar places. They make white shirts glow quite brightly, and teeth and fingernails too.

Trivia: The reason shirts glow so brightly under black light is that producers of laundry detergent add chemicals that glow under UV. This way, when one is outside, the natural UV from sunlight causes these chemicals to glow, making the shirt whiter than white.

There's a number of materials that fluoresce under black light. The most common are:

All materials either reflect or absorb light at distinct wavelengths or colors. Any absorbed light turns into energy and this energy is then emitted back out of the object, usually as infrared because the radiation is produced by chaotic motion. However, some materials produce some of the radiation less randomly and therefore some of the wavelengths of the light they emit are higher and therefore might be visible. When light of one wavelength is absorbed and its energy is then emitted at a lower but still visible wavelength, the material doing the absorbing and emitting has the property of fluorescence.

A black light that shines its invisible high-energy light on something fluorescent gives that thing the energy it needs to emit visible light.

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