ANALYSIS OF LIGHT BY A PRISM SPECTROMETER
Introduction
In this lab you will calibrate a prism spectrometer using known wavelengths and use the results to
determine the wavelengths of selected lines from atomic hydrogen and mercury. In future labs
you will learn how to determine wavelengths using a meter stick and diffraction grating.
Equipment
Gaertner-Peck brand spectrometer
Spectrum tube power supply
Spectrum tubes (hydrogen, helium and mercury Incandescent lamp
The components of the spectrometer are indicated on the drawing below.
(Illustration courtesy of Central Scientific Company. Reprinted with permission.)
You will find that the spectrometer has been adjusted for proper operation. Please do not change
the position of the prism or the height of the prism table. Each person using the instrument will
probably need to make some adjustment of the ocular (eyepiece lens) to accommodate for his or
her eye. You will want to have both the image of the illuminated slit and the crosshairs in focus.
You will use the helium spectrum tube to provide data for constructing a “calibration curve” for
the instrument. This is a graph of wavelength (vertical axis) as a function of deviation angle.
(This angle is defined below.) Then when you look at the spectra of other sources, you can find
the wavelengths by locating the appropriate angle on the calibration curve. Before you collect
data, you should be familiar with the procedures for locating and centering the image of the slit,
and for reading the angle from the vernier scale.
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The scales are divided into a bottom (regular) and top (vernier) parts. Look where the 0 line is
with respect to the regular scale and this tells you degrees - to the half degree ("30 minutes" since
1 degree = 60 minutes). In the examples below, you have 161 degrees and 161 degrees + 30
minutes.
The vernier scale divides each half-degree interval into thirty one minute intervals. Where the top
(vernier) scale best lines up with the bottom scale (see illustrations) you get the number of
minutes. You must convert the angle in degrees and minutes into decimal degrees. Two
examples are shown below. You may find it helpful to illuminate the scale with the incandescent
lamp.
PROCEDURE
1. Insert the helium discharge tube in the power supply. (Caution! The high voltage electrical
connections to the ends of the tube are recessed for your protection. Do not insert or remove a
tube while the power is on. After a short period of operation the tube gets quite warm. When you
change tubes, allow a little time for a tube to cool down before you remove it.) Turn on the
supply and make sure that the slit is aligned with the tube. The clamp screw for fine motion (F)
must be loosened before you try to rotate the collimator housing. You can look for the helium
spectral lines by rotating the collimator housing out of the way and looking into the side of the
prism for the light path labeled “to eyepiece” in the drawing.
D
From slit
To eyepiece
2. Now find the first (longest wavelength) red line of the helium spectrum. Line it up with the
crosshairs as well as you can, then tighten the clamp screw (F) and turn the adjusting screw (J)
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for fine motion to center the line. Read the vernier scale and enter the value in the data table.
Repeat this for the remaining helium lines in the table. You will not use all of the lines but just
the brightest. The template can be used as a guide. The template is a live document so the curve
will be updated as you write over the previous values. Turn off the spectrum power supply but
do not remove the helium tube.
The calibration data will look something like the following.
Calibration Curve
Spectrometer Angle Decimal Deviation Wavelength
Degrees Minutes Degrees Angle
nanometers
Red
129
20
129.33
50.67
706.5
Red
129
7
129.12
50.88
667.8
Yellow
128
29
128.48
51.52
587.6
Green
127
16
127.27
52.73
501.5
Blue Green
127
8
127.13
52.87
492.2
Blue
126
42
126.70
53.30
471.3
Violet
126
6
126.10
53.90
447.1
Color
3. EXCEL will draw the calibration curve as a continuous line. Do not change the scale or size of
the graph. Note that the axes are wavelength (vertical axis) vs. angle. (The relationship between
wavelength and angle is not simple!) If you do not get a reasonably smooth curve, identify
those points that seem questionable and retake the data. Your values for the spectral lines of
hydrogen and mercury will not be good if your calibration curve is poor.
4. Replace the helium tube with the hydrogen tube. Check the alignment of the tube and the slit.
Record the angles for the specified hydrogen lines. Repeat this procedure for the mercury tube.
ANALYSIS
Print out the calibration curve and use it to find the wavelengths for the specified hydrogen and
mercury lines. Record these in the table. Calculate and record the percent difference in each case.
Report
In your report, you might address the following questions.
1. What is the accuracy of your measurements? Are all percentage differences too high or too
low? What are some of the experimental difficulties that give rise to the differences between
measured and accepted values?
2. The textbooks state that the visible spectrum ranges from about 400 nm to about 700 nm. How
consistent is this with your observations of these line spectra?
3. Does the refractive index of the prism material increase or decrease as the wavelength of the
incident light increases? What experimental evidence is there for this? (Look at the drawing that
defines the deviation angle D and your calibration curve.)
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