Question

A ray is reflected in turn by three plane mirrors mutually at right angles to each other what is the angle between the incident and reflected rays

Answer 1

The light is reflected straight back in the direction of the incident ray. In other words, the light rays undergo a rotation through 180 degrees. So the angle between the incident and reflected ray is 180 degrees (or zero degrees, depending on how you define things).

Answer 2

The light is reflected straight back in the direction of the incident ray. In other words, the light rays undergo a rotation through 180 degrees. So the angle between the incident and reflected ray is 180 degrees (or zero degrees, depending on how you define things).