PASCO SP-9268A STUDENT SPECTROMETER - Manual (Page 5)

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012-02135F Student Spectrometer
®
COLLIMATOR
SLIT
RED LIGHT
TELESCOPE
COLLIMATOR
LIGHT
SOURCE
PARALLEL BEAM
DIFFRACTION GRATING
(OR PRISM)
GREEN LIGHT
ANGLE OF
DIFFRACTION
EYE PIECE
Introduction
In principle, a spectrometer is the simplest of scientific
instruments. Bend a beam of light with a prism or dif-
fraction grating. If the beam is composed of more than
one color of light, a spectrum is formed, since the vari-
ous colors are refracted or diffracted to different angles.
Carefully measure the angle to which each color of light
is bent. The result is a spectral "fingerprint," which car-
ries a wealth of information about the substance from
which the light emanates.
In most cases, substances must be hot if they are to emit
light. But a spectrometer can also be used to investigate
cold substances. Pass white light, which contains all the
colors of the visible spectrum, through a cool gas. The
result is an absorption spectrum. All the colors of the vis-
ible spectrum are seen, except for certain colors that are
absorbed by the gas.
The importance of the spectrometer as a scientific instru-
ment is based on a simple but crucial fact. Light is emit-
ted or absorbed when an electron changes its orbit within
an individual atom. Because of this, the spectrometer is a
powerful tool for investigating the structure of atoms. It's
also a powerful tool for determining which atoms are
present in a substance. Chemists use it to determine the
constituents of molecules, and astronomers use it to de-
termine the constituents of stars that are millions of light
years away.
In its simplest form, a spectrometer is nothing more than
a prism and a protractor. However, because of the need
for very sensitive detection and precise measurement, a
real spectrometer is a bit more complicated. As shown in
Figure 1, a spectrometer consists of three basic compo-
nents; a collimator, a diffracting element, and a tele-
scope.
The light to be analyzed enters the collimator through a
narrow slit positioned at the focal point of the collimator
lens. The light leaving the collimator is therefore a thin,
parallel beam, which ensures that all the light from the
slit strikes the diffracting element at the same angle of
incidence. This is necessary if a sharp image is to be
formed.
The diffracting element bends the beam of light. If the
beam is composed of many different colors, each color is
diffracted to a different angle.
The telescope can be rotated to collect the diffracted
light at very precisely measured angles. With the tele-
scope focused at infinity and positioned at an angle to
collect the light of a particular color, a precise image of
the collimator slit can be seen. For example, when the
telescope is at one angle of rotation, the viewer might
see a red image of the slit, at another angle a green im-
age, and so on. By rotating the telescope, the slit images
corresponding to each constituent color can be viewed
and the angle of diffraction for each image can be mea-
sured. If the characteristics of the diffracting element are
known, these measured angles can be used to determine
the wavelengths that are present in the light.
Figure 1 Spectrometer Diagram