Spherical Reflector
The blue arc is part of a spherical reflector. The applet shows how light is reflected from it. Each time light strikes the reflector it obeys the law of reflection (angle of incidence equals angle of reflection--these angles are measured using the tangent lines to the arc where the light strikes). You can change the angle of incidence by moving point C. You can change where the light hits the mirror by moving the yellow point on the mirror, You can change the size of the mirror by dragging one of its blue end points.