Two plane mirrors are inclined at an angle `theta` . A light ray incident on one of the mirrors undergoes multiple reflections. The angle of incidence on the first mirror is `15^@` . After reflection from the second mirror, if the light ray retraces its path, determine the value of `theta.`

Explanation

`angle OAB = 90 - 15 = 75^@ ` (angle of incidence = angle of reflection )

In `Delta OAB , angle OAB + angle AOB + angle ABO = 180^@`
` 75 + theta + 90 = 180^@`
` rArr theta = 180 - 165`
` = 15^@` (angle light ray retraces its path only if it incident along the normal )