If the angle between the incident ray and the reflected ray is 100^o then find the angle between the reflected ray and the surface of the mirror.

According to the law of reflection of light, 
Angle of incidence = Angle of Reflection, 
where, the angle of incidence is the angle between the incident ray and the normal, and the angle of reflection is the angle between the reflected ray and the normal.

In the given example,

The angle between the incident ray and reflected ray= 100^o
Therefore, the angle of reflection= 50^o
The angle between the reflected ray and the mirror= 90-50= 40^o

Hence 40^o is the correct answer.