Question

According to law of reflection, How this statement(angle of incidence = angle of reflection= 90 degree) is correct? how both equal to 90 or 0 degree?

Answer 1

Physical laws don't have proofs. They have experimental evidence.
A and B are two points on the same side of the mirror. A ray of light from A reflects off the mirror and goes to B. If I understand correctly, what is to be proved is this: among all paths going from A to the mirror and then to B, the shortest one is the one for which the two angles are equal.

When the angle of incidence is 0 degree, it simply means that it is overlaping normal ray and its angle with the normal ray is 0 degree. So as per law of reflection - angle of incidence will be equal to angle of reflection. So angle of reflection will be 0 degree and so it will return back to the same path.

When angle of incidence is 90 degree then angle of reflection will also be 90 degree, which means that the light ray will get reflected along the same path but it means that the incident ray and reflected ray will join to form a straight line.