File: FRANHFER.DOC

FRAUNHOFER LINES

Background

A rainbow is good example of a spectrum. It is created by a spectrometer which uses your eye as detector, the sun as a source and raindrops as the dispersive element. If one looks at a spectrum of the sun in more detail, one sees that all wavelengths do not have the same intensity. Furthermore, some wavelengths are missing. These missing wavelengths, which appear as many thin dark lines, are called Fraunhofer lines and are caused by the atmosphere of the sun absorbing some of the light which the sun’s surface is producing. In order to see these lines a small point or line source must be used because the sun by itself is too large to produce the high resolution spectrum necessary.

References

1. Crummet, W. P. and Western, A. B., University Physics (Modes and applications), (Wn. C. Brown, Publishers, Dubuque, IA, 1994) pp 1060-1064.

2. Hewitt, P.G. Conceptual Physics, 7th Ed., (Harper Collins College Publishers, 1993) pp 527, 551-552.

3. Serway, R., Principles of Physics, (Saunders College Publishing NY, 1992) pp 343-348.

Materials

Diffraction grating

Instructions

This experiment is carried out in bright sunlight. The problem is to find some object that will reflect only part of the sun’s image

For this experiment find a shiny silvery "pole". Examples of such "poles" are television antenna supports, Ham radio antennas, and shinny automobile parts such as chrome strips.

After finding a suitable shinny silvery "pole" position yourself such that you can see the reflection of the sun in the "pole". You may have to select a different time of day if the geometry of the sun, "pole", and the observer will not allow one to see the reflected sunlight from the "pole".

If all else is in order, view the reflected sunlight with the diffraction grating used previously. The diffraction grating may need to be rotated so the long axis of the reflecting object is parallel the lines in the diffraction grating.

Special note. If you do this experiment in a car parking lot where there are many reflections, caution will need to be used to identify the particular spectrum that goes with a particular source. Even on one car there may be multiple reflections visible at the same time. Extending an imaginary arrow from the red end of the spectrum through the blue end to the source will enable the source to be identified.

1. Record as best you can what you see, noting the color of the light of the spectrum. Was there anything unusual? What?

2. Sketch out the geometry of the way in which the observation was made and include the time of day.

After this has been done, find an object that has a rather large blob of reflected sunlight such as the side of a car or a car windshield and carry out the same observation.

3. Is there any difference? What?

Report

The notes and sketches describing what was observed, or not observed, should be turned in as the report for this experiment.