Magnifying glass?

Question

You've seen how you can burn things with a magnifying glass and some sunlight before right? Well my question is what would happen if you did that under black light (alone)?

Answer 1

Black light is an incorrect term used for ultra-violet light. Ultra-violet light focused through a magnifying glass would produce some heat, but a great deal less than sunlight.

Answer 2

The real problem is that the sources of black light (UV) are so diffuse that even with a large magnifying glass, you don't get much energy in one spot. And some lenses don't pass UV. The sun is far away and there are parallel rays (nearly) hitting the lens and forming a tight little spot of IR (heat) and light. Most black light sources on earth are coated filters on incandescent or fluorescent lamps and thus the magnifying glass produces an image of a blob or a line.
To really see an effect you would need to put a UV filter on a high intensity stage lighting spot light. And then you might only get a brighter area of black light effects like glow in the dark.