PASCO OS-8539 Educational Spectrophotometer - Manual (Page 23)

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M o d e l N o . O S - 8 5 3 7 a n d O S - 8 5 3 9 A c t i v i t y 2 : A b s o r p t i o n ( D a r k L i n e ) S p e c t r u m
19
Activity 2: Absorption (Dark Line) Spectrum
EQUIPMENT NEEDED
Introduction
The purpose of this activity is to determine the wavelengths of the colors absorbed by a liquid sample.
Theory
One of the most important applications of spectrophotometers is
to identify substances by their absorption spectra. For example, it
is possible to identify tiny amounts of sodium dissolved in a com-
plicated liquid (such as beer) because sodium has a unique
absorption spectrum.
An incandescent source such as a hot solid metal filament pro-
duces a continuous spectrum of wavelengths. A substance placed
in the path of light from a continuous spectrum source will
absorb certain colors from the continuous spectrum. The individ-
ual colors that are absorbed appear as gaps or “dark lines” in the
otherwise continuous spectrum (Fig. 2.1).
Procedure
In this activity, the High Sensitivity Light Sensor measures the relative intensity of colors of light in an continu-
ous spectrum produced by an incandescent light. Then, the sensor measures the relative intensity of colors of
light in an absorption spectrum produced when light from the incandescent source passes through a liquid sam-
ple. The Rotary Motion Sensor measures the angle,
, of each part of the continuous spectrum and then the
absorption spectrum.
The data acquisition program records and displays the light intensity and the angle. You can use the program’s
built-in data analysis tools to find the angle for each “gap” or dark line in the absorption spectrum, and then you
can determine the wavelength,
.
Spectrophotometer System (OS-8539)
or
Spectrophotometer Kit (OS-8537) High Sensitivity Light Sensor (CI-6604)
Rotary Motion Sensor (CI-6538) Basic Optics Bench (part of OS-8515)
Aperture Bracket (OS-8534)
and
Incandescent Light Source, DC, regulated Large Rod Stand (ME-8735) (2)
Rod, 45 cm (ME-8736) (2) Colored Liquid Sample (about 5 mL)
PASCO Interface Data acquisition software
light ray
light ray
Grating
Cuvette
Cuvette with
sample
continuous
spectrum
absorption
spectrum