Question

If humans cannot see ultraviolet waves how can ultraviolet light be used to gather evidence of a crime

Answer 1

Ultraviolet light is invisible but it gets absorbed by body fluids like blood or semen and can be re-emitted as visible light. In a darkened crime scene, body fluids will show up and can be collected and analysed.

Answer 2

Ultra violet detects the presence of radioactive substances. Ultra violet light causes substances to fluoresce and give off visible light.