NAME:____________________________
Astronomy 106 Observing Spectra with a Diffraction Grating
This exercise is required and will count 30 points toward your final grade. To receive full credit
it must be turned in at the beginning of class on Wednesday, 14 February.
• Carefully read the lab and complete the experiment as instructed. If something is not clear,
do not hesitate to ask the instructor.
• Each student’s work must be his/her own.
• Neatness counts, information which is difficult to decipher will not receive credit.
From the instructor, obtain a spectroscope. Be very careful NOT TO TOUCH THE
SURFACE OF THE GRATI NG! Fingerprints will degrade its performance. The diffraction
gratings are a sheet of clear plastic which has had over ten thousand grooves per inch inscribed
on the surface. The grooves create the equivalent of very narrow silts on the surface of the
plastic. Because of the nature of light acting as a wave, light passing through a narrow gap is
spread out into a rainbow as different colors are spread (dispersed) by an amount related to the
color. In the language of physics, the dispersion is proportional to the wavelength of the light.
The net effect of all the slits acting together breaks the light into a spectrum.
Each rainbow band will contain the colors from red to orange, yellow, green, blue, and violet
(ROYGBIV). Other wavelengths of light are produced beyond the red (infrared) and beyond the
violet (ultraviolet), but the human eye is only sensitive to wavelengths of light between roughly
400 and 700 nanometers. Each person’s eyes differ in sensitivity, it is very possible that you do
not observe the same colors as other people. An extreme case of decreased sensitivity to color
perception is known as color blindness. Look more carefully, beyond the first spectrum, you
should see a second fainter spectrum like the first, and possibly a third beyond the second
spectrum. The additional spectra contain no information not also found in the primary spectrum,
and need not be considered.
Observing Exercise:
Use the spectroscope to observe many different type of light sources, with the goal of finding
one good example of each of the ten types of light source listed on the following page. The
sources are grouped by the overall color of their appearance, as either basically white lights, red
lights, yellow lights or blue/green lights. Note that a source can appear to be reddish colored
when it produces its light (as observed with your spectrograph) in a completely different way
from another reddish source.
To distinguish between different types of light sources requires that you carefully note the
appearance of each spectrum. Include a count of the emission lines, the wavelength of the lines
as measured by the spectrograph’s scale, and a concise description of the brightness of the
emission lines. Be sure to stress the features that make one spectrum different from the others.
The light source you investigate should be the dominant source of brightness in the
immediate area. It works best if you are in an otherwise darkened room when observing indoor
light sources. Make outdoor observations at night, or in surroundings which are darkened except
for the light source in question.
