Diffraction Lab
Purpose
To measure the size of a gap of an optical disc using diffraction.
Background
When waves pass through a openings that are small relative to the wavelength of a wave, the waves will bend as they pass through the opening, the bending is called diffraction. The longer the wavelength in relation to the gap size, the more bending will occur. This is one behavior of light that can not be explained by thinking of light as a particle! When two or more gaps are side by side, the waves that leave them will overlap and create an interference pattern as can be seen in this diagram.
Image by Lacatosias / CC-BY-SA 3.0
The relationship between the variables in this setup is as follows:
λ = dx/L
λ = wavelength of the light
d = spacing between lines in the grating
x = separation between the spots
L = distance from the grading to the spot on the screen
You may have noticed that when you look at an optical disc you see rainbows. This is because optical discs have very small grooves on their surface. The grooves are acting to diffract the incoming light, and making an interference pattern. Each color of light has a different wavelength, and as such a slightly different pattern - the rainbows you see.
Task
Design an experiment to measure the gap in optical discs and then compare what you find to known values. Hint: You will be reflecting light off the surface rather than passing it through the gaps, but the equation works the same. And monochromatic light like what is given off of a laser is easiest to use so you know the light’s wavelength.
Grading Rubric:
Section | Poor | Fair | Excellent |
Procedure ___10pts | The procedure is very vague, a person trying to replicate it may as well not look at it. | The student’s procedure has one or two vague items, but could still be carried out by somebody who is familiar with what is trying to be done. | The student’s procedure is very specific. Lists all materials and describes the setup clearly. |
Data and Calculations ___10pts | The student has data from just one trial. A calculated value is shown, but no work is shown with it. | The student has data that is a little hard to follow from multiple trials. There is an example calculation but it isn’t clearly labeled. | The student has clearly labeled data from multiple trials. The students has at least one example calculation with all work shown and labeled for all calculated values. |
Conclusion and Evaluation ___10pts | The student compares value to uncited value. Percent error is not given. The evaluation is vague, and not specific to their procedure. | The student compares their value to an uncited literature value. Percent error is given.The evaluation gives one issue that came up specifically during the lab investigation. | The student has a comparison of their calculated value to a cited literature value. The percent error has been calculated. An evaluation that addresses specific lab issues that could lead to the error are addressed. |
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