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

Ultraviolet light is invisible to human eyes, but it can make some substances fluoresce: they absorb ultraviolet energy and then emit visible light. This can reveal traces such as certain bodily fluids or other materials at a crime scene. Choice A describes this effect; showing warm and cold areas is associated with infrared imaging, not ultraviolet light.

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