Deviation of light ray by plane mirror

Steps:

• The incident ray strikes the mirror at an angle of incidence $i = 40^\circ$ to the normal.
• The reflected ray leaves at the same angle $i = 40^\circ$ to the normal on the other side.
• The total turn in direction, or deviation $\delta$, is $180^\circ - 2i$.
• Substituting $i = 40^\circ$: $\delta = 180^\circ - 80^\circ = 100^\circ$.

Why C is correct:

• By the law of reflection, deviation $\delta = 180^\circ - 2i$, so for $i = 40^\circ$, $$.

Why the others are wrong:

• A: Equals $2i = 80^\circ$, the angle between incident and reflected rays, not the deviation.
• B: Occurs if $i = 45^\circ$ ($\delta = 90^\circ$), not matching the given angle.

Final answer: C