Experiment No. 2
APh 24
Spring 2012
Diffraction Grating and Monochromator
I. Brief Overview
The objective of this experiment is for you to experimentally study the basic
equations derived for the diffraction grating and its use in a practical
spectrometer.
II. Prelaboratory Preparation
Review Hecht sections (multi-slit interference, diffraction grating, and blazing
gratings) 10.2.3, 10.2.6, and 10.2.8 (pp 478 in particular) and your class notes and
problem sets from APh 23 on gratings.
Do the following in your notebook:
1. (20 pts) A transmission diffraction grating has 500 lines/mm. A He–Ne laser
( = 632.8 nm) is directed through the grating with an angle of incidence  Plot
the diffraction angles for the +1 and –1 orders as  varies from 0 to 90 degrees.
2. (20 pts) In your own words, describe what advantage a blazed grating has
(Hecht 478). How is the blaze angle related to the angle of incidence?
3. (20 pts) A He–Ne laser (~632.8 nm) emits at frequencies spaced by 750 MHz.
How large does the diffraction grating from problem 1 need to be in order to
resolve the frequencies (set m = 1 and i)?
4. (30 pts) You learnt from APh23 about the grating formula that is a convolution
of a single slit diffraction function and a multiple wave interference fringe
function (Hecht page 461, Eq. 10.31). Use your own words, describe a) what
determines the spectral resolution using Rayleigh criteria; b) what determines the
diffraction angle; c) what determines the dispersion, i.e. angular
deflection/wavelength, and d) the significance of total illumination area on the
grating. These are the considerations in designing a grating-based spectrometer.
5. (10 pts) Find examples of spectra for typical incandescent lamps and mercury
vapor lamps.
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III. Procedure
Note: There are several parts to this experiment, so it is important to pace
yourself. You should take no more than 1 hour for part A and part B each, and 2
hours for part C.
Caution: under no circumstances that one should directly look into a laser beam!
As a general rule, you should not touch optical surfaces with your fingers.
A. Transmission Diffraction Grating (estimated time - 50 min)
In this part you will verify the grating equation experimentally and measure the
diffraction efficiency of a transmission grating.
1. (10 pts) Set up a mirror and transmission grating (the one that looks like a
clear circle of glass) in an arrangement similar to Figure 1. Ask TA to show you
how to adjust the laser beam so that it is perpendicular to the wall. Using the
transmission diffraction grating, position it so that the diffracted spots appear on
the strip of graph paper on the wall at some convenient distance away. Adjust the
grating for normal incidence. (What is an easy way to tell that you are at normal
incidence?)
2. (20 pts) Determine the diffraction angles 1, , +2, -2,··· from the distances
between the orders projected on the wall. Describe your measurement and
calculation. Include the distance measurements in your notebook. Estimate the
grating spacing in line/mm. From the variation in the values for grating spacing
you obtained from the various diffraction angles, estimate the accuracy of your
value for the grating spacing.
3. (20 pts) With the optical power meter, measure the power diffracted into the
various orders Pm (-2 ≤ m ≤2) when the beam is at normal incidence. The power in
the diffracted orders is very low, so you will have to turn out the room lights.
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Calculate the diffraction efficiency of each of the orders :
Pm
PIncident . Is there
variation of intensity in the various orders as you change the angle of incidence?
You only need to observe the variation qualitatively. From your observations
would you say that this grating is blazed (Hecht pp 478)?
B. Reflection Diffraction Grating (estimated time – 50 min)
4. (25 pts) Set up the mirror to reflect off the gold–coated infrared grating in a way
similar Figure 2. CAUTION! This grating is a very expensive item. PLEASE DO
NOT TOUCH THE GRATING SURFACE! Tilt the beam upward with the turning
mirror so that the reflections from the grating clear the top of the mirror. Observe
the many orders. Identify which reflection is the zero order reflection so that you
can adjust for normal incidence. Describe a simple measurement which uses the
grating’s rotation stage and its reflected zero order to measure/estimate the first
order diffraction angle. (Don't forget the spots on the screen move by 2 when the
stage rotates by .) Estimate the first order diffraction angle using your technique
and estimate the line/mm of the grating.
5. (25 pts) Rotate the grating off normal incidence and observe the motion and
intensity of the various orders. Are any significantly stronger or weaker? Which
orders and under what conditions? Measure the efficiency of this grating in its
best (strongest) order. How does this grating’s best efficiency compare to the
transmission grating in part A? Estimate the blaze angle. Make a sketch of what
you think the grating grooves may look like, showing numerical values for the
scale and angle of the blaze.
C. B&L monochromator and spectroscopy (estimated time - 100 minutes)
There are two B&L monochromators on the table. One is for visual inspection
(unit A) and one is for doing spectroscopy. Check with instructor or TA before you
proceed.
6. (20 pts) Read the “Principle of high intensity monochromator” on the table.
Carefully remove the covers from the monochromator and the attached lamp
source. The grating is on the cover piece, so be careful. Observe the various parts.
Make a sketch in your notebook showing the entire optical path including lamp,
lenses, slit, and grating. Provide a brief description of each component in the
optical path.
7. (20 pts) Exam the grating on the cover piece, the mechanism for tuning the
grating, and the wavelength calibration. Carefully rotate the wavelength tuning
knob and observe wavelength reading and the corresponding amount of angular
rotation of the grating. Estimate the grating dispersion and the grating property
(lines/mm). Remember this is a reflective grating. Replace all covers back
carefully.
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8. (20 pts) Now move to the monochromator for spectroscopy. Set both entrance
and exit slits to 1 mm. Place the small incandescent lamp about 3.5” from the
condenser lens. Adjust the lamp height level with the center of the lens and the
entrance slit of the monochromator. Turn on the lamp. Place a screen before the
exit slit. Turn the wavelength-tuning knob and observe light out from the exit slit.
Sketch and describe what you observe. You will need to dim all room lights to
observe some parts of the spectrum.
9. (30 pts) Place the photo-detector right behind the exit slit of the
monochromator. Ask a TA about the use of the power meter and its wavelength
calibration. A copy of the response curve of the detector is in the lab. You will need
it to correct the detector’s relative power reading. Record the spectrum (Power vs.
wavelength), about 10-20 points across the entire monochromator wavelength
range. Plot the directly observed spectrum. Replot with the detector response
correction. Compare it with the spectrum you find in your prelab #5.
10. (30 pts) Remove the incandescent lamp and replace with the vapor discharge
lamp provided. The discharge lamp emits strong UV light. Do not directly look into
the discharge lamp. Use the paper shield as soon as the alignment is completed.
Use the cover provided to avoid direct exposure to the lamp. Ask a TA about how
to turn on the discharge lamp. Turn on the lamp. Tune the wavelength dial slowly
and observe the deflection of power meter reading. Find all spectral line
wavelengths and record their power reading at the detector. Use the detector
response curve, plot the relative intensities of the spectrum line. Compare it with
the mercury lamp spectrum you find in Prelab #5.
What is the FWHM of the strongest spectral line? What is the likely limiting
factor for the linewidth? Now reduce both slits to 0.5 mm. Re-measure the FWHM
and discuss the result.
11. Turn off the lamp and laser. Clean up the lab and leave all parts as you find
them before you leave. (You will suffer penalty points if you don’t.)
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