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'Y'Z', as shown. As a result the new reflected ray is at an angle α from the original reflected ray. Then:

When two plane mirrors are placed perpendicular to each other (at an angle of 90°), any light ray that undergoes reflection from both mirrors will emerge parallel to its original incident path. This is a fundamental property of a corner reflector.

• The total deviation produced by two successive reflections is given by the formula: Deviation = 360° - 2θ, where θ is the angle between the mirrors.
• Since the mirrors are perpendicular (θ = 90°), the total deviation is 360° - 180° = 180°.
• A deviation of 180° means the reflected ray is always antiparallel (parallel but in the opposite direction) to the incident ray, regardless of the angle of incidence.

When the entire mirror assembly is rotated by an angle θ, the angle between the mirrors remains 90°. Consequently, the final reflected ray remains parallel to the fixed incident ray. Therefore, there is no change in the direction of the reflected ray, making α = 0.