The refraction of light through a triangular glass prism is depicted in the illustration above. At the first surface AB, a beam of light PE is penetrating the glass from the surrounding air. Due to refraction, the light ray EF has been bent in the direction of the normal. The light ray FS has penetrated from the glass to the air at the second surface AC and has been bent away from the typical path. Angle of deviation is the angle formed by extending the incident ray and intersecting it with the emergent ray.