Diffracting Gratings

Description of the Process Used to Take the Photographs

First, an audio CD was placed on top of two layers of black construction paper. The CD-paper system was then placed underneath a lamp (white light) and photographed. The camera flash was not used as it tended to make the CD look ‘fuzzy’ and ‘blurred’.

Physical Concepts and Principles Involved in the Image

This photograph uses a compact disc to illustrate the phenomena of interference of light waves.

The typical compact disc is an example of reflection grating. It is composed of closely spaced parallel lines and it has a reflective aluminum coating. This structure allows the disc to create interference patterns of light. When light (of any kind) is shone onto the grating, the closely spaced parallel lines (and aluminum) will reflect and diffract the light back towards its source. Often, there are more than 10000 lines/cm so that the diffraction/reflection pattern can be easily observed.

White light was used to produce the interference pattern found in our photograph. If monochromatic light were used, only one color with specific maximums and minimums would be reflected back towards the observer. White light, however, is made up of many different wavelengths with each forming its own maxima, creating a colorful pattern in the process. Apparently white light is also supposed to produce a large and white central maximum near the center of the pattern. However, this maximum did not properly appear in our photograph.

Historical Background of This Phenomenon

The first diffraction grating was constructed in 1820 by German optical worker, Joseph von Fraunhofer. He used long parallel wires to create the closely spaced lines, and as a result it only worked effectively with longer wavelengths.

Since Fraunhofer, diffraction gratings have continued to be improved by famous scientists such as Rutherford (who created one 2 inches thick), Rowland, and Brashear.

The compact disc, which made use of the reflection grating, was mass developed by Sony in the early 1980s.