On a moving plane mirror, an incident ray makes an angle of 60° with the…

The law of reflection states that when light reflects off any surface, the angle of incidence and the angle of reflection — both measured from the normal to the surface at the point where the ray strikes — are always equal. This equality holds at the very instant of reflection, independent of whether the reflecting surface itself is stationary or in motion; a mirror's own motion can shift the position or apparent velocity of the image it forms, but it does not alter this fundamental angular relationship.

Here, the incident ray strikes the plane mirror making an angle of 60° with the normal. Since the mirror is only translating (moving), and not described as being tilted or rotated between the instant of incidence and reflection, the law of reflection applies exactly as it would for a stationary mirror: the reflected ray also makes an angle of 60° with the normal.

Contrasting the other offered values against this rule:

• 90° would require the reflected ray to always leave perpendicular to the incident ray as a fixed rule — but reflection has no such fixed right-angle rule; the reflected angle is set entirely by the incidence geometry, not by a constant 90°.

• 30° results from mistakenly halving the given incidence angle, perhaps by confusing the angle measured from the normal with the angle measured from the mirror's surface (its complement) and then applying an unnecessary halving step.

• 120° is the total angular separation between the incident ray and the reflected ray about the point of incidence — i.e., twice the angle each ray makes with the normal — which answers a different question (the angle BETWEEN the two rays) than the one asked here (the reflected ray's own angle from the normal).

So, applying the law of reflection directly, the reflected angle is 60°, unaffected by the mirror's motion.