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Rainbows, Absorption Lines, Spectra
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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