This resource is set up for students to investigate how the angle of incidence affects the angle of refraction. It is easier to use than using a plastic protractor and makes this practical far easier (especially for foundation groups). Simply place a semi-circular block on the ‘D’ and aim a beam of light (from a ray box) at the central cross. Both the angle of incidence and angle of refraction are then easy to identify. This can also be used to identify the ‘critical angle’.