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course Phy 242
7/21 9 am
Experiment 29: InterferenceUsing a hand-held laser pointer and a diffraction grating consisting of
lines on a rectangular transparency, we observe the maxima created when the
light is directed through the pattern at various separations, and with
various incident angles. We determine the angular separation of the maxima
and use this separation to estimate the wavelength of the light. We then
use sets of parallel straight lines on the same transparency to determine
the wavelength of the light.
Stapled to the paper rulers in your lab materials package is a rectangular
transparency a few inches on a side. The transparency contains copies of
various patterns of lines.
In at least one pattern the lines form a V.
Orient the pattern so that the V is upright, with the widest spacing at the
top.
Move at least 3 and preferably 5 or more meters from a smooth wall. Shine
the laser through the V near the top of the pattern and observe the image
made by the light on the wall. Measure the distance from the transparency
to the wall.
12 ft
Gradually move the laser down through the V, so that it shines between
lines that move progressively closer and closer together. Observe what
happens to the pattern on the wall.
The one dot seperates slightly into more dots in a vertical or horizontal
line
Continue moving down the V until you obtain the most distinct possible set
of bright spots on the wall.
Note the vertical position of the beam on the V.
About halfway through the 2nd largest V
As best you can, determine for this position the average distance between
the distinct bright spots formed on the wall.
about 3-4 mm apart
Measure the width of the V at this point, and the number of spaces between
the threads across the width.
1.4 cm, the spaces are too small to see but I estimate about 30 spaces
Record also the distance to the wall.
12 ft = 3.66 m approx
There are also a few rectangular patterns consisting of parallel lines.
The spacing of the lines varies from rectangle to rectangle.
Repeat the preceding exercise using different rectangular grids.
For each grid determine the average distance between the bright spots on
the wall, the average distance between the grid lines and the distance from
the plastic rectangle to the wall.
grid 1: 1 mm , lines about 1 mm apart
grid 2: 2-3 mm , lines about .5 mm apart
grid 3: >1 mm , lines about 2 mm apart
According to your results, how is the spacing between the bright spots on
the wall related to the distance between the lines?
According to my results, the spacing between the bright spots on the wall
increases as the distance between the lines decreases.
What is the ratio of the spacing between the dots to the distance between
the plastic rectangle and the wall?
grid 1: 1 mm / 3660 mm = 2.73 e^-4
grid 2: 2.5 mm / 3660 mm = 6.83 e^-4
grid 3: .1 mm / 3660 mm = 2.73 e^-5
What distance is in the same ratio with the spacing between the lines?
Grid 2 where the distance between the dots is about 3 mm, is the best ratio
Red light usually has a wavelength of 650 nm, therefore this ratio of 6.83
e^-4 makes sense.
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You report that on grid 2,
2-3 mm , lines about .5 mm apart
I assume the 2-3 mm is the distance between the spots on the wall 12 feet away.
With the .5 mm grid the usual observation is that the dots on the wall are somewhat further apart than that. However, we can for the moment accept the 2-3 mm measurement.
I agree that the ratio of that spacing to the distance between the grid and the wall is about 6.8 * 10^-4.
Now, what is the spacing between the lines, and what distance would be in this ratio to that spacing?
How does this distance compare to the 650 nm wavelength of the light?
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Just submit the above questions with your answer(s).
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