Question

what do you mean that the incident ray, the normal and the reflected ray all lie in the same plane​

Answer 1

The law of reflection states that the incident ray, the reflected ray, and the normal to the surface of the mirror all lie in the same plane.
Furthermore, the angle of reflection is equal to the angle of incidence . Both angles are measured with respect to the normal to the mirror.The law of reflection also holds for non-plane mirrors, provided that the normal at any point on the mirror is understood to be the outward pointing normal to the local tangent plane of the mirror at that point. For rough surfaces, the law of reflection remains valid. It predicts that rays incident at slightly different points on the surface are reflected in completely different directions, because the normal to a rough surface varies in direction very strongly from point to point on the surface. This type of reflection is called diffuse reflection, and is what enables us to see non-shiny objects.

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