OPTICS ANIMATION / SIMULATION: light rays reflecting off mirrors.
Light rays reflecting off a rotating plane mirror. Note how the angle of incidence (of the ray striking the mirror) is the same as the angle of reflection (of the ray leaving the mirror). The normal (line at right angles to the plane of the mirror) is shown in purple, so that these angles are obvious for the middle ray.
Light rays reflecting from a curved mirror that changes curvature. Simulated beams of light reflect from a parabolic mirror that goes from concave to convex. A convex parabolic mirror diverges parallel rays to create a virtual focus behind the mirror. A concave parabolic mirror brings parallel rays to a focus in front of the mirror. The ability of concave parabolic reflectors to concentrate parallel rays to a single focus is applied in reflecting telescopes (which use huge parabolic mirrors to collect distant starlight). Satellite dishes (and radio telescopes) also use parabolic collectors to collect radio waves (the detector is put at the focus).