Shown in the figure are two plane mirrors XY and YZ (XY ⊥ YZ) joined at their edge. Also shown is a light ray falling on one of the mirrors and reflected back parallel to its original path as a result of this arrangement. The two mirrors are now rotated by an angle θ to their new position X'YZ', as shown. As a result the new reflected ray is at an angle α from the original reflected ray. Then :

Explanation:

• When two plane mirrors placed at 90° to each other then, the incident ray and the reflected ray are antiparallel to each other.
• Angle between the incident ray and reflected ray is 180° 
• As shown in the figure are two plane mirrors XY and YZ (XY ⊥ YZ) joined at their edge.
• Also shown is a light ray falling on one of the mirrors and reflected back parallel to its original path as a result of this arrangement.

• The two mirrors are now rotated by an angle θ to their new position X'YZ', as shown.
• As a result, the new reflected ray is at an angle α from the original reflected ray.
• Then α = 0°, because, after rotation of the mirror combination, reflected rays are parallel to the incident rays,
• So, the new reflected ray is parallel to the original reflected ray.

Additional Information

• When two plane mirrors are placed at an angle other than 90 degrees to each other, the incident ray and reflected ray are no longer antiparallel, and their relationship is determined by the angles of incidence and reflection for each mirror.
• The angles formed by the reflected rays can be different from each other and will depend on the angle between the mirrors.