caution when examining sunlight; do not
look directly into the Sun.
Discussion:
Refer to the discussion on Analytical
Spectroscopes for information on how
diffraction gratings produce spectra.
Glass prisms are heavy. The more
separation one wants for the wavelengths,
the thicker the glass needs to be. Grating
spectroscopes can do the same job but are
much lighter. A diffraction grating can
spread out the spectrum more than a prism
can. This ability is called dispersion.
Because gratings are smaller and lighter,
they are well suited for spacecraft where
size and weight are important
considerations. Most research telescopes
have some kind of grating spectrograph
attached. Spectrographs are
spectroscopes that provide a record,
photographic or digital, of the spectrum
observed.
Notes:
- Most science supply houses sell diffraction
grating material in sheets or rolls. One
sheet is usually enough for every student
in a class to have a piece of grating to
build his or her own spectroscope.
Holographic diffraction gratings work best
for this activity. Refer to the note on
sources in the previous activity.
- Many light sources can be used for this
activity, including fluorescent and
incandescent lights and spectra tubes with
power supplies. Spectra tubes and the
power supplies to run them are expensive.
It may be possible to borrow tubes and
supplies from another school if your school
does not have them. The advantage of
spectrum tubes is that they provide
spectra from different gases such as
hydrogen and helium.
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For Further Research:
- Using colored pencils or crayons, make
sketches of the spectrum emitted by
different light sources. Try incandescent
and fluorescent lamps, bug lights, street
lights (mercury, low-pressure sodium, and
high-pressure sodium), neon signs, and
candle flames. How do these spectra
differ?
- How do astronomers measure the spectra
of objects in space? What do those
spectra tell us about these objects?
- Relate this activity to the Analytical
Spectroscope activity that follows (page
34).
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