QUESTION #72

# How tall does a mirror have to be to fit an entire persons body? Does distance from the mirror play any type of role?

Asked by: Eric Anderson

### Answer

You can best find this answer with a newspaper and roll of tape on a full length mirror in your home. Cover all the parts of the mirror you DON'T need to see your reflection. You will see that you only need a mirror length of about 1/2 your height.

This is due to the principle of reflection, which states that the angle of incidence equals the angle of reflection. A light ray from your foot strikes the mirror like a ball off a wall. Thus, by striking the mirror half way between your foot and eye, the ray never interacts with that part of the mirror below the half way mark. Since that half way point doesn't change as you move away from the mirror, the useful area of the mirror remains the same, regardless your distance from it.

Another way of looking at this is to imagine your image is another person standing on the other side of a window and the same distance away from it. To see their feet, the bottom of the window need not be to the floor but only along a line connecting your eye with their feet. A full length mirror need not be a doorway, only a window.

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor