Question

If the incident ray falls on a surface normally, then the angle of reflection is _________ degree.
(A) 90
(B) 0
(C) 30
(D) 60

Answer 1

Hint: The angle of incidence and the angle of reflection are both calculated with respect to the normal drawn to that surface and not with respect to the plane of the surface itself. Also, the angle of incidence is equal to the angle of reflection irrespective of the shape of the reflecting surface.

Complete Step by step solution:
By the law of reflection, it is said that the angle of incidence and the angle of reflection for any ray of light, reflecting off of a surface will always be equal, irrespective of the surface being plane as in the case of plane mirror or curved as in the case of concave and convex mirrors or even in the case of arbitrarily shaped reflecting surfaces. As such the formula is given which is $\angle i = \angle r$where ‘i’ is the angle of incidence or the angle at which the light ray strikes the surface and ‘ r’ is the angle of reflection or the angle at which the reflected ray leaves the surface.
One thing which should be kept in mind is that these angles of incidence and reflection are always measured from the normal to the surface.

The diagram shown above is an instance of reflection. Here ‘N’ is the normal to the reflecting surface ‘M’. ‘1’ shown here is the ray of incidence which forms $\angle i$ with ‘N’ and ‘2’ is the reflected ray which leaves at $\angle r$ with ‘N’. Here $\angle i$ and $\angle r$ will both be equal due to the law of reflection.
As for the question above, the incident ray falls on the surface normally which means that ray ‘1’ will coincide with ‘N’ and as such, will make an angle of ${0^ \circ }$with the same. Thus, by the law of reflection, the reflecting ray that is ray ‘2’ will also make the same angle with the normal which is ${0^ \circ }$ and that ray too will coincide with the normal.

Note:
In case of the curved surfaces such as concave and convex reflecting surfaces or in case of arbitrarily shaped reflecting surfaces, first thing that needs to be done is to plot the tangent at the point of reflection and then perpendicular to the tangent, a normal must be drawn before plotting incidence and reflection rays and angles.