Why Do Black Lights Make Things Glow?

Although human eyes cannot see the ultraviolet light emitted by black light bulbs, certain objects glow under black light. This occurs because those objects reflect that light back as part of the visible spectrum.

Black Light Bulbs

Black light bulbs glow a dim violet color when used in a lamp. Although the bulbs actually emit a great deal of light, human eyes cannot see it because the light's frequency is too high.

Fluorescence

Fluorescence is the ability of some objects to absorb ultraviolet light, then emit it as light of a lower frequency---light that human eyes can see. Fluorescent objects glow in black light. Other objects do not.

Fluorescence in Laundry Detergent

Laundry detergents that claim to "make whites whiter" contain phosphors that will make white clothing appear whiter in normal light and glow in black light.

Fluorescence in Money

New $20 bills have a thin security thread in them. This thread will glow under black light, providing another way of confirming that the bills are not counterfeit.

Fluorescence in Bananas

Ripening bananas will glow blue under black light, because some of the products that break down chlorophyll in the ripening process are fluorescent.

References

• Ask the Van, University of Illinois Physics Department
• Science Daily
• SciFun