CHAPTER INVESTIGATION
Observing Spectra
OVERVIEW AND PURPOSE Visible light is made up of
different colors that can be separated into a rainbow band called
a spectrum. Astronomers gain information about the characteristics
of stars by spreading their light into spectra (spectra is the plural
form of spectrum). A spectroscope is a device that produces
spectra. In most spectroscopes, diffraction gratings are used to
separate light into different colors. The colors with the longest
wavelengths appear farthest from the slit in a spectroscope. The
colors with the shortest wavelengths appear closest to the slit.
In this investigation you will
• build a spectroscope and observe the spectra of three
different light sources
• identify ways in which the spectra of light sources differ
Procedure
MATERIALS
• shoebox with lid
• ruler
• scissors
• diffraction grating
• tape
• index card
• pencils or markers in a
variety of colors
• incandescent light
• fluorescent light
for Challenge:
• cellophane in several
colors
684 Unit 6: Space Science
Cut a hole measuring 3 cm by 1.5 cm in
each end of a shoebox. Make sure that
the holes line up.
On the inside of the box, tape a piece
of diffraction grating over one of the
holes. Handle the diffraction grating by
its edges so that you do not get fingerprints on it.
step 1
Cut an index card in half, and tape the halves over
the outside of the other hole as shown. Leave a very
narrow slit between the two halves of the index card.
Put the lid on the shoebox. Then turn off the
overhead lights in the classroom.
Look through the hole
covered with the
diffraction grating,
aiming the spectroscope’s slit at the sky
through a window.
Caution: Never look
directly at the Sun.
Observe the spectrum
you see to the left of the slit.
2. ANALYZE The shorter the wavelength of a
color, the closer it appears to the slit in a spectroscope. On the basis of your observations,
which color has the shortest wavelength?
Which color has the longest wavelength?
3. INFER How might the spectra look different
if the slit at the end of the spectroscope were
curved instead of a straight line?
INVESTIGATE Further
step 5
Repeat step 5 while aiming the spectroscope
at an incandescent light and then at a
fluorescent light.
Observe and Analyze
Write
It Up
CHALLENGE Cover the slit on your spectroscope with a piece of colored cellophane. Aiming
the spectroscope at a fluorescent light or another
light source, observe and draw the resulting
spectrum. Then repeat with cellophane of other
colors. List the colors that each piece of cellophane transmitted. Did these results surprise
you? If so, why?
1. RECORD OBSERVATIONS For each
light source, draw in your data table the
spectrum you see to the left of the slit.
Describe the colors and patterns in the
spectrum, and label the light source.
2. IDENTIFY LIMITS What problems, if
any, did you experience in observing the
spectra? Why was it important to turn off
overhead lights for this activity?
Conclude
Write
It Up
tra
Observing Spec
nalyze
Observe and A
Sources
of Different Light
Table 1. Spectra
Light Source
Drawing
Description
1. COMPARE AND CONTRAST How did
the spectra differ from one another? Did you
notice any stripes of color that were brighter
or narrower than other colors in the same
spectrum? Did you notice any lines or spaces
separating colors?
Conclude
Chapter 19: Exploring Space 685