Today we’re looking at a simple but powerful demonstration of optics, Demo L2-01: Optical Board and Plane Mirror.
A bright white light source is directed through a baffle with several slits, producing a set of rays. Lenses are used to collimate these rays, and they are then reflected off of a long plane mirror. If the lenses are adjusted such that the incoming rays are approximately parallel, the reflected rays will be as well.
Unlike light diffracted through a lens or prism, reflected light from a surface is unaffected by the frequency of the light. The reflection off of a flat mirror is dependent only on the angle the light strikes at. This demonstration shows that the angle of incidence is equal to the angle of reflection.
You can experiment with this at home – just find any flat mirror (there’s often one on the wall above the sink), shine a light on it with a flashlight or phone, and measure how the reflected light moves when you move the light.
To see this in mathematical detail, check out this simulation at oPhysics: https://www.ophysics.com/l9.html . You can see how light rays from the tips of an object will reflect off a surface. Try dragging the object (a big blue arrow) around to see how the reflection moves. The simulation can also trace out virtual rays – the light rays “behind” the mirror that aren’t really there, but are the geometric extensions of the reflected rays. Following the virtual rays lets you predict where you eye would “see” the reflected image of the object being.