In the diagram, a light ray PO strikes a vertical mirror at point O, and the reflected ray is OQ. By projecting an imaginary line through point O perpendicular to the mirror, known as the normal, we can measure the angle of incidence, θ i and the angle of reflection, θ r. The law of reflection states that θ i = θ r, or in other words, the angle of incidence equals the angle of reflection.