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

DESCRIPTION: Gwen is a bit perplexed about her understanding of the image of a plane mirror. Perhaps you are also. Q: Is your image in a plane mirror inverted (a) top-to-bottom, (b) left-to-right, (c) front-to-back, or (d) none of the above? A: (c) 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 left-to-right inversion! Ask whether you see your front or your back in the mirror when you look at yourself. Point out that 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.

Gwen is much happier now that she fully understands the image of a plane mirror, as seen if you click your mouse on her picture above.

SUGGESTIONS: See Question of the Week #18 for information on using this demonstration to enhance class involvement.

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.

SETUP TIME: None.