# Refraction and Lenses - Mission RL4 Detailed Help

 In the diagram at the right, a light ray is shown passing from medium 1 to medium 2. The angle of incidence for this ray is represented by angle ____. (Careful!! Pay attention to the choices.)
 Definition of Angle of Incidence: The angle of incidence is the angle made by the ray of light approaching a surface (incident ray) and the normal line drawn to the surface at the location where the ray contacts the surface.
 A common mistake made by beginning physics students is to believe that the angle of incidence is the angle between the incident ray and the boundary. But don't be fooled! Angles of incidence (and refraction) are always measured between the ray of light and the normal line.
 The diagram shows two media (shaded pink and yellow), a solid black boundary line, a normal line (dashed) and an incident and refracted ray (red). The letters are used as labels to represent the various angles between the normal line, light ray, boundary, etc. These letters on the diagram do not necessarily relate to the letters of the answers. For instance, angle D may correspond to choice E. Be careful.