Question

Why can convex lens burn a paper using sunlight

Answer 1

Convex lenses concentrates the light energy to one spot on the paper so that the heat energy accumulates on that one small spot of paper. As the heat increases, combustion will occur when the spot becomes too hot and the paper will burn.

Answer 2

Convex lens are also called converging lens because it converges the light. Convex lens can concentrate the sun's rays onto a small area, heating up the area and thus resulting in ignition of the exposed surface.