Question

A convex lens focuses Sunlight on white paper and black paper kept at focus which would start burning first?

Answer 1

The black paper will burn first.

According to the principle of Black Body Radiation by Gustav Kirchhoff, any black coloured body will absorb and emit maximum amount of heat, compared to other bodies of different colours.

When the convex lens focuses sunlight on the black paper, it will absorb more heat as compared to the white paper, kept at the same focus because of the above principle.

Answer 2

Black Paper would burn First.
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Reason:

1. Convex lens is called as converging lens which focus all parallel beam of sunrays at Focus.

2. When sunlight is made to incident on an object , any of the following three things will happen
---> Reflect
---> Transmitt
---> Absorb

3.When light is absorbed, the object will heat up .

4.Hence in this case black paper will absorbs maximum light energy and heats up faster as compared to white paper which will reflect some of the incident radiation .

5. White colour is a good reflector of heat . Good reflectors is a bad absorber of heat. so white paper takes some more time to burn.