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What is wrong with this picture?

A Physics Riddle

What is wrong with this picture?

The picture shows a pyramid-shaped prism dispersing white light into the classic rainbow of colors. There are two problems with the picture. First, the dispersion direction is incorrect. The prism will disperse the light in a vertical direction if the white light hits it as shown, not in the horizontal direction. Second, the color bands of the rainbow should not be the same width. The majority of the frequencies in the visible spectrum appear reddish to our eyes. That is, the red band is the widest band. In contrast, the yellow band is very narrow. The table and colored plot below categorize the colors by wavelength (assuming the light is traveling in air). The plot is a bit misleading because our eyes are not equally sensitive to all the colors. The sensitivity peaks right near the green-yellow wavelengths and falls off at the red and violet wavelengths. As a result, the violet and red bands in a rainbow will not usually appear as wide as shown in the plot simply because we don't see the longer red wavelengths and shorter violet wavelengths as well as the other wavelengths. A prism forming a rainbow is often used in advertising, but seldom is the formation shown correctly! This picture was obtained from the standard clip art directory in Microsoft PowerPoint (ver. 4.0).

Color Wavelength Range (nm) Red 622 - 780 Orange 597 - 622 Yellow 577 - 597 Green 492 - 577 Blue 455 - 492 Violet 390 - 455

This cover photo shows a magnified view of the operation of a semiconductor quantum well laser. The laser was fabricated at the Institute of Optics at the University of Rochester as part of my graduate research in visible semiconductor lasers under the advisement of Dr. Gary Wicks.

The laser consists of a thin active layer (about 100 Angstroms) of Gallium Indium Phosphide sandwiched between several layers of Aluminum Gallium Indium Phosphide. The active layer is so thin (about 50 atomic layers) that the electrons and holes trapped in the layer behave as if they were inside a square, one-dimensional, quantum well. Hence, these devices are called quantum well lasers. The crystal structure was grown in a Molecular Beam Epitaxy (MBE) machine operated by Michael Koch. Once the material was grown, I fabricated individual, rectangular lasers by defining metal electrical contacts and cleaving out end mirrors.

The photo shows a top view of the laser. The laser is 0.03 millimeters wide and 0.3 millimeters long. Current is injected into the top by a metal probe. (The presence of the probe causes the break in the red line around the top edge of the laser in the photo.) Red light with a wavelength of 676 nanometers is being emitted out of the two ends of the laser. The striations in the output light are caused by the light reflecting off of scratches in the copper block on which the laser is sitting.

Semiconductor lasers are used extensively in optical communication and in optical storage devices such as CD players. This particular laser was the first visible quantum well laser grown in an MBE machine that utilized solid phosphorus as a source.

Background courtesy of me!