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

DESCRIPTION: The phrase "CHOICE MATERIAL GLASS OXIDE" with CHOICE .......... OXIDE red and .....MATERIAL GLASS..... blue is viewed through a glass rod, which clearly inverts the blue words while leaving the red words erect. The pictures above show the printing viewed directly (top picture) and the writing viewed through the glass rod (bottom 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 just above the red letters, as seen in the photograph below. Because your finger is below the letters in the image, your students might conclude that the red letters are in fact upside down, and figure out what happened to their minds.

SUGGESTIONS:

See Question of the Week #12 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.