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Name: Greg
Status: educator
Grade: 9-12
Location: N/A
Country: N/A
Date: 2/1/2005


Question:
Why doesn't a rainbow show absorption lines? How can you get our sun's light to show these lines? Do you need special equipment?


Replies:
I assume you are referring to the Fraunhofer lines. They are not visible (I do not think) in a rainbow because there is enough light scattering that they would be obscured. In order to view them you need to construct a basic spectrograph. The web site:

http://www.physics.nmsu.edu/research/lab110g/html/AFRANHFER.html

gives one way of doing this, but I am sure there are others. The incoming light has to be a narrow strip -- that is what the shiny pole in the web site above does. This has to be parallel to a diffraction grating (you can probably get an inexpensive one from a science supplier like Edmunds Scientific, and preferably a darkened room so stray light does not mask the effects. Some assembled spectrographs are available that also may work and they range from inexpensive to pricey, depending upon the bells and whistles. If there is a university near you with an astronomy department, you may be able to borrow or use a pretty good instrument just for the asking.

SAFETY: In the course of developing the experiment of course do not look directly at the Sun. I know that may seem condescending but I do not mean it to be, just want to err on the side of safety.

Vince Calder


To see absorption lines, you need a device or a scattering process that puts each different wavelength at a particular angle, and this is not what happens in a rainbow. The light refraction that produces a rainbow smears each wavelength across a large angular range (from zero to the angle at which you actually see that wavelength). The intensity of different wavelengths peaks at different angles, but at any particular angle there will be light of many different wavelengths. The absorption lines actually are there, but their effect on light intensity is very small because the "missing" light is such a small fraction of the total light.

You can see absorption lines with an entrance slit and a diffraction grating or prism.

Tim Mooney



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