PURPOSE: Counterintuitive investigation of the nature of the image of a plane mirror.

DESCRIPTION: Q: Is your image in a plane mirror inverted (a) top-to-bottm, (b) left-to-right, (c) front-to-back, (d) all of the above, or (e) none of the above? A: Front-to-back only.

This possibly counterintuitive result can be investigated using three small coordinate axes, one left-handed and two right-handed. Position one right-handed coordinate system in front of the mirror and ask a student to line up the second right-handed coordinate system so that it looks like the image in the mirror. It will quickly be seen to be impossible. Try again with the left-handed coordinate system. That this can be done indicates that the mirror inverts one of the axes, but which one? Everyone agrees that the mirror does not invert top-to-bottom. Stand in front of the mirror and wiggle your right hand; the hand on the same side wiggles in the mirror, indicating no inversion! Ask whether you see your front or your back in the mirror when you look at yourself. Point out that an you only see your front because the image has been inverted front-to-back.

A plane mirror produces only one inversion: front-to-back. This is called a perverted image.


REFERENCES: (PIRA 6A10.37) See Demonstration Reference File for articles discussing this weirdness.

EQUIPMENT: Plane mirror on stand, two sets of right-handed coordinate axes and one set of left-handed coordinate axes.


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