Question

Why paper catches fire when a convex lens I used to focus sunlight?

Answer 1

A convex lens can be used to focus the parallel rays of sunlight passing through it, into a singular point. 

If this focal point is aimed accurately onto a piece of paper, then you will see a small imageof the sun at the focal point, and this is where the paper will begin to burn.

Answer 2

Answer:

when the convex lens is used to focus sunlight, on a piece of paper, it focusses sunlight at its focal point on the paper. Since intense sunlight passing through the lens is focussed at its focal point on paper, it heats the paper up at that point and starts burning.

Explanation:

•The sunrays are taken as coming from infinity distance towards the convex mirror.

•According to the rules for drawing ray diagrams for convex lens, When light rays coming from infinity pass through the convex mirror, they get refracted and converge at the focal point of the lens.

•If a piece of paper is kept in such a way that the focal point of the convex lens falls on the paper, the high energy sun rays are concentrated at the focal point of the lens at the paper.

•This results in heatingbof the paper at that point, so much so that it starts to burn.