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course Phy 232
As I stated in my lab, it is really hard to get accurate data in this experiment. I even had someone holding the laser pointer while I took measurements, and it was still hard to get precise results. Data will therefore be skewed.
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.
• 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 red dot on the wall becomes more and more blurry until it finally hits a point where it breaks into several dispersed, bright spots.
• 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.
The position is approximately 0.6 cm from the bottom of the v.
• As best you can, determine for this position the average distance between the distinct bright spots formed on the wall.
It was very hard to determine the actual average distance between the dots because it was very hard to keep the laser completely still, but it was observed that the distance was approximately 1.5mm
• Measure the width of the V at this point, and the number of spaces between the threads across the width.
Width of v is 1.3 cm. There are 8 spaces.
• Record also the distance to the wall.
Distance to wall 4m
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.
I couldn’t get some of the grids to work but for the ones I got
Grid # Average Distance Between Bright Spots Average Distance Between Grid Lines Distance From Rectangle to Wall
1 0.1 cm 0.05cm 4m
2 0.12cm 0.02cm 4m
3 0.1cm 0.1cm 4m
4 0.2cm 0.04cm 4m
According to your results, how is the spacing between the bright spots on the wall related to the distance between the lines?
It was extremely difficult to get accurate measurements in this lab. Not only was it hard to measure the distance between the dots, but it was also extremely difficult to measure the distance between line on the plastic grid as well.
This being said, it appears from my data that the smaller the distance between lines on the grid, the greater the separation distance of the dots on the wall.
What is the ratio of the spacing between the dots to the distance between the plastic rectangle and the wall?
In pretty much all the cases, the ratio is approximately 0.001m/4m=0.00025
What distance is in the same ratio with the spacing between the lines?
The distance between the dots must be in the same ratio with the distance between the lines, I believe, but my there is too much error associated with my data to be entirely sure.
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If the ratio is .00025 and the distance between the lines is .05 cm, then the quantity which is in the same ratio with the distance between the lines is
.00025 * .05 cm = .0000125 cm = .000000125 or 1.25 * 10^-7 m, about 125 nanometers.
This would be your estimate of the wavelength of the light.
What estimates would you get from the other observations?
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