How do you get things to glow under a black light?

Answer 1

Black light is only black because very little of the light the lamp produces is in the visible spectrum. Most is actually ultraviolet (UV) light. The frequency of such light is just a little higher than violet (hence the name ultraviolet).

The human eye cannot detect UV light, so black light is mostly invisible to us (you might see a faint violet glow however).

UV light can cause certain materials to glow due to an effect called Fluorescence. This entails a material absorbing light at a certain frequency (e.g. UV light) and retransmitting it at another frequency, for example visible light. We cannot see UV light but we can see the retransmitted visible light causing the object to glow.

Black light theaters make good use of such materials, but in many countries the paper money also contains fluorescent materials. That is why people working behind the counter in shops will sometimes hold money under a UV lamp; counterfeited money might not have such security features and will thus reveal itself for the fake it is.