A ray of light is incident on a plane reflecting surface at an angle of 30°. Find the deviation in the incident ray. What will be the deviation if the ray suffers a reflection again at a surface inclined at 60° to the first surface?

240°

$∠B'CM_2 = 60°, ∠M_2CD = 60°, ∠B'CD = 120° = D_2, ∠DCN_2 = ∠N_2CB =30°$

$∠ABN_1 = ∠N_1BC =30°; ∠CBA' = 120° = D_1$

The incident ray of light AB on the first mirror M₁ at an angle of incidence of 30° is reflected along BC at an angle of reflection 30°.

∴ Deviation in incedent ray = 60° + 60° = 120°

The reflected ray BC is incident on the second mirror M₂.

Since $∠M_1OM_2 = 60° , ∠BCO = 60°$,

$∴ ∠BCN_2 = ∠N_2CD = 30°.$

∴ Deviation D₂ of the ray = 60° + 60° = 120°

∴ Total deviation due to the two mirrors = $D_1 + D_2 = 240°$