PASCO OS-8539 Educational Spectrophotometer - Manual (Page 19)

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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 1 : E m i s s i o n ( B r i g h t L i n e ) S p e c t r u m
15
Activity 1: Emission (Bright Line) Spectrum
EQUIPMENT NEEDED
Introduction
The purpose of this activity is to determine the wavelengths of the colors in the spectrum of a mercury vapor
light.
Theory
An incandescent source such as a hot solid metal
filament produces a continuous spectrum of wave-
lengths. Light produced by an electric discharge in
a rarefied gas of a single element contains a lim-
ited number of discrete wavelengths - an emission
or “bright line” spectrum. The pattern of colors in
an emission spectrum is characteristic of the ele-
ment. The individual colors appear in the shape of
“bright lines” because the light that is separated
into the spectrum usually passes through a narrow
slit illuminated by the light source.
A grating is a piece of transparent material on
which has been ruled a large number of equally
spaced parallel lines. The distance between the
lines is called the grating line spacing, d.
Light that strikes the transparent material is diffracted by the parallel lines. The diffracted light passes through
the grating at all angles relative to the original light path. If diffracted light rays from adjacent lines on the grating
interfere and are in phase, an image of the light source can be formed. Light rays from adjacent lines will be in
phase if the rays differ in path length by an integral number of wavelengths of the light. The first place that an
image can be formed is where the path length between two adjacent light rays differs by one wavelength,
.
However, the difference in path length for two adjacent light rays also depends on the grating line spacing, d, and
the angle,
, at which the two light rays were diffracted by the grating.
The relationship between the wavelength of the light, , the grating line spacing, d, and diffraction angle, , is as
follows:
= d sin 
In the diagram (Fig. 1.1), the path length for Ray A is one wavelength longer than the path length of Ray B.
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) Rod, 45 cm (ME-8736) (2)
Mercury Spectral Tube and Power Supply Large Rod Stand (ME-8735) (2)
PASCO Interface Data acquisition software
Grating
Figure 1.1: Ray diagram for first order diffraction pattern
Ray B
Ray A
light rays
d = grating line
spacing
path difference =  = d sin 
 = angle of
diffraction