PHY124
Lab # 12
Diffraction Gratings
Name ____________________
Lab Partners
Purpose: To study the diffraction of light using a diffraction grating.
Apparatus
1 Laser Diode Pointer
1 Diffraction (Project Star) Grating
1 Optical Bench (with screen and support clamps)
1 Metrestick or ruler
Procedure
1) Place the screen in a holder at the 115 cm location on the optical bench..
2) Place the laser pointer in a support clamp at the 0 cm location on the optical bench.
PLEASE NOTE: Anyone caught fooling around with the laser pointers will immediately be
kicked out and will get a grade of zero for this lab.
3) Place the diffraction grating in a support clamp at the 105 cm location on the optical bench.
(This gives a distance of 10 cm between the screen and the diffraction grating.)
4) Adjust the laser pointer in the support clamp so that it is giving off light and that the light
beam passes through the diffraction grating.
5) At this time three red dots of laser light should show up along a horizontal line on the screen.
If they fall along a vertical line, take the diffraction grating out of the support clamp and rotate
it 90 degrees and put it back into the support clamp. If the three dots do not all fall on the
screen adjust the positions of the support clamps in the bases so that they do.
6) Using the metrestick, measure the distance between the central dot and the two other dots. (If
these distances are not the same it means that the screen is not parallel to the diffraction
grating and parallel to the optical bench. If this happens, simply adjust the support clamp
holding the screen until the distances from the central dot to the other two dots are the same.
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7) Record the distances from the central dot (zeroth-order fringe) to the next set of dots (the first
order fringe) in the table provided.
8) Reset the support clamp for the diffraction grating so that it is at the 100 cm location on the
optical bench. (This gives a distance of 15 cm between the screen and the diffraction
grating.)
9) Repeat the measurements described in steps 6 and 7.
10) Reset the support clamp for the diffraction grating so that it is at the 95 cm location on the
optical bench. (This gives a distance of 20 cm between the screen and the diffraction
grating.)
11) Repeat the measurements described in steps 6 and 7.
12) Reset the support clamp for the diffraction grating so that it is at the 90 cm location on the
optical bench. (This gives a distance of 25 cm between the screen and the diffraction grating.)
13) Repeat the measurements described in steps 6 and 7.
L
distance between
diffraction grating
and screen

angle formed
between central ray
and first bright dot
s
distance between
central image and
first bright dot
D
distance between
lines etched into the
diffraction grating
10 cm
15 cm
20 cm
25 cm
14) Calculate the distance between the lines etched into the diffraction grating. (Please enter
these into the table provided above.) Please note: that the wavelength of light given off by
the laser pointers is listed on the laser pointers themselves. Since it is given as a range of
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possible frequencies, simply use the center frequency of the range listed.
Questions for Lab # 12
Diffraction Gratings
1) Were the four calculated distances between the lines etched into the diffraction grating close
to each other? (Determine the average of the calculated distances between the lines and the
relative error between each of these and the average found. Remember: close means within 5%.)
2) What would happen to the distance between the bright images formed from the interference
pattern of a diffraction grating if you could increase the number of lines per cm etched into the
diffraction grating?
3) What happens when a small beam of white light is passed through a diffraction grating?
Briefly explain why this happens.
4) How is the operation of a diffraction grating different from a prism? (Give a brief explanation
of why each of them does what it does to the light going through them.)
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