Reflection in mirrors

The paper, mirror, ray box, protractor, card and pins are simulated, and the rays are drawn exactly, so the angles of incidence and reflection come out exactly equal and the image exactly 6.0 cm behind the mirror and 3.0 cm long. The angles 20°, 40° and 60° are the model’s settings.

A real ray box gives a beam about a millimetre wide that spreads out a little, a real protractor can be read to about 1°, and pencil marks are never perfectly placed, so real readings of i and r often differ by a degree or two, and dashed extensions cross over a small region rather than at a point.

Most real mirrors are glass with a silvered back, so the light reflects a few millimetres behind the front of the glass, and you draw the mirror line along the back. Here the mirror reflects exactly on the line. In the view from above in part 1, the mirror does not show the rays on the paper, so no ray is seen going on behind the mirror before images are met in part 2. The search pin is 8 cm tall and the mirror 4.2 cm, so its top shows above the mirror next to the image of the object pin. The view in the mirror is worked out by the computer.

With reduced motion each step happens at once: the rays, the pencil lines and the readings appear straight away as they are at the end of the step, and the camera cuts between the view from above and the eye low in front of the mirror.