This is about physics....?

Question

suppose you would like to start a fire by focusing sunlight onto a piece of paper, what type of spherical mirror should the paper be held for best results?

Answer 1

The first source shows the shape that is needed. It is a special shape of a concave mirror.This is the shape that can concentrate parallel light rays from an far distance to one point above the dish. The larger the dish the more light energy that is concentrated. Read this information. The more you understand what is going on the better you can do what you want to do.

The first and third items on the second source are probably the best sources for these dishes that I can find.

Answer 2

concave

Answer 3

A concave mirror could do the job just fine but you can use a convex lens for better result.

(It might take some time to concentrate enough heat energy to burn the paper)