The angle between the emergent
ray RS and the direction of the incident ray PQ
is called the angle of deviation, d In the quadrilateral AQNR, two of the angles
(at the vertices Q and R) are right angles Therefore, the sum of the other angles of the
quadrilateral is 180° ÐA + ÐQNR = 180°
From the triangle QNR,
r1 + r2 + ÐQNR = 180°
Comparing these two equations, we get
r1 + r2 = A
(9