7.A ray of light incident normally on a plane mirror.What are the values of glancing angle and angle of deviation?

When a ray of light incident normally on a plane mirror it is reflected normally back in the original path. As we know that glancing angle is the angle between incident ray and plane mirror which is 90^o in the given case. ... Since the angle of deviation for a plane mirror is twice the glancing angle, So the angle of deviation is 180⁰