Example 1: Light enters a parallel-sided glass block at A. The angle between the side of the block and the ray of light in air is 35°. The light then strikes the edge of the block at B. The figure below shows the ray of light and the glass block. The angle of refraction in the glass at A is 33°. What is the angle of incidence at the surface A? θair = 90° - 35° = 55°