Arc lamp and mirror on large disk rotate showing beam angles.

Arc lamp, with mirror on large rotateable disk showing beam angles. This apparatus is designed to show that the angle of incidence equals the angle of reflection. White light from a carbon arc shines through a slit, producing a ray that reflects off a front-surface mirror mounted on a large white rotateable disk marked with degrees. The ray also glances against the disk, making it visible to the class.