Question

why does obtaining the image of sun on a paper with the help of concave mirror burn the paper

Answer 1

while obtaining the image a lot of heat comes to a fixed point due to the light energy which burns the paper

Answer 2

Because concave mirror focus the sunlight at one particular point . At that particular point , amount of sunlight is converged and heat is trapped at that point . This is because , it burn the paper on which the image is focussed .