PURPOSE: To stimulate thought about the variation of the index of refraction with color.

DESCRIPTION: The phrase "GREEN TOMATO" with "GREEN ..." printed in green ink and "... TOMATO" in red ink is viewed through a glass rod, which clearly inverts the green word left-to-right while leaving the red word normal. The pictures above show the printing viewed directly (left picture) and the writing viewed through the glass rod (center picture).

Those who are familiar with the dispersion of light by a prism might immediately jump to the conclusion that this effect is due to the variation of index of refraction (and therefore focal length) with wavelength. Further reasoning can be used to conclude that if this is so, the focal points are different for each color, and therefore the magnifications should be different.

Ask your students to guess why this unusual behavior occurs. Suggest a variety of implausible reasons, such as dispersion of light as suggested above, some difference in the rod in front of various letters, or polarization.

As a hint, hold your finger to the side of the red letters, as seen in the photograph at the right above. Because your finger is on the opposite side of the letters in the image from the object, your students might correctly conclude that the red letters are in fact inverted, and figure out what happened to their minds.

SUGGESTIONS:

See Question of the Week #205 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for a multitude of variations as well as other important information on this very significant and thought-provoking demonstration.

EQUIPMENT: Pre-assembled as photographed.

SETUP TIME: None.