Diffraction
Name: _______________
Copyright R Mahoney 2010
>> 101 students: do only steps 1 through 8. <<
The purpose of this lab is to explore diffraction and the principle of superposition.
Using a diffraction grating and a pen laser, we will measure the angle made by
the first off-axis bright spot (fringe). From this information, we'll calculate the
wavelength of the laser's light beam.
Recall from class that we determined that for two or more slits, the first bright
spot is given by the formula:
n * λ = d * sinΘ ,
where n is the order of the (major) bright spot (for this lab, n = 1), λ is the
wavelength of the coherent laser light, d is separation distance between
"slits" in the diffraction grating, and Θ is the angle made between the
center (or zeroth) bright spot on the screen or wall where the pattern is
formed, the "center" of the diffraction grating, and the nth bright spot on
that same screen or wall.
Procedure
1. With the pen laser, test which orientation of the diffraction grating gives
a vertical diffraction pattern.
2. Choose where you wish to image the diffraction pattern (for example
the nearest wall). You'll want your screen to be relatively far away (for
theoretical reasons, and for precision and accuracy of measurement).
3. Using masking tape, tape the diffraction grating to the side of your work
table with the orientation needed to produce a vertical diffraction pattern.
The grating's center does not have to be aligned with the table's edge: just
make sure that any diffraction pattern produced is more or less vertical on
the wall. Rest the pen laser on the table so that its beam outlet is
perpendicular to the diffraction grating. Turn on the laser to see the
diffraction pattern on the wall.
4. Using one of the tape measures provided, measure the horizontal
distance from the diffraction grating to the wall. Horizontal is where the
zeroth bright spot is located. Place a piece of masking tape where that
zeroth bright spot is. Any unit of distance will do, so use inches.
Horizontal distance: __________ in.
5. With the laser beam on and the diffraction pattern showing, one person
in your lab group should use masking tape to mark the first bright spot up.
Now measure the vertical distance between the zeroth and first bright
spots. Vertical distance: __________ in.
6. Use the measurements above to find the tangent, and then later the
sine of the angle corresponding to the first bright spot. This is theta (Θ).
tanΘ = __________
sinΘ = __________
7. Determine the distance d between markings on the diffraction grating.
This is the inverse of the number of rulings per unit length. Express d in
nanometers. d = __________ nm
Use the formula given above to determine the wavelength (λ) of the light
coming out of your laser pen. λ = __________ nm.
8. Observe the range of wavelengths given on the side of the laser pen.
Does your reading fall within this range?
9. Since there are red and green lasers, if you used a green laser, try
repeating this experiment with a red laser, or vice-versa. What are your
results?
10. Try using the glass plate with green edges to create other diffraction
patterns. Specifically, use the two slit portion of the plate.
How does the diffraction pattern for the diffraction grating compare to the
two slit diffraction pattern?
What kind of diffraction do you get if you use a wide-spaced double slit?
11. How might you improve the precision of your wavelength
measurement?