You are looking down at the top edge of a plane mirror. The yellow box represents a light ray. The law of plane mirror reflection tells us that the light ray will be reflected off the plane mirror at an angle equal to the angle at which the light ray struck the mirror. Use the slider to set the angle you would like to rotate the mirror counter-clokwise from its horizontal position. The left end of the mirror is pivoted at the black square. To what angle(s) must the mirror be rotated so that the light ray hits the red target?
Rotating Plane Mirror - an exercise in plane geometry. The law for plane reflection states that the angle at which an object reflects from a plane surface equals the angle at which the object strikes the surface. What if the direction of the light source is kept constant but the mirror rotates? When the Green Flag is clicked the variable "angle" rotates the plane reflecting surface (seen in a Top View) counter-clockwise through "angle". The yellow square, representing a light ray, is always facing up (0º). It moves in a straight line until if strikes the reflecting surface. The correct formula for reflecting the ray of light off the mirror's surface is turn right (180º – 2 x angle) (see the script for sprite 5). Verify this formula by applying the angle-sum of a triangle = 180º theorem.
