Question

a ray is reflected in turn by 3plane mirrors mutually perpendicular to each other .the angle between the instant and reflected ray (a)90°(b)60°(c)180°(d)none of these​

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).