Experiment 12: CD Diffraction

In this experiment we used a laser with a known wavelength to measure the width of the grooves in a compact disk. To do this we first set up the experiment with a laser at an angle of about 45 degrees to the normal of the surface of the disk. As the laser became hit the surface of the CD the laser beam would diffract and split into multiple beams of light. The reason why we decided to place the CD at a 45 degree angle was so the the diffracted beams could be shown a white board that was facing parallel to the laser beam before hitting the CD. The picture below better illustrates the apperatus.

Using this setup we measured the distance from the CD to the white board to be 0.070 meters +/- 0.0005. We also know that the wavelength of the laser to be 670 nm +/- 0.10 and that the distance from the central maximum to the first maximum is 0.028 m +/- 0.0005. Using the equation y=[m*(lambda)*x]/a where x = 0.7 meters, lambda = 670*10^-9 meters, m = 1, and y = 0.028 meters we can solve for (a) (the slit spacing in the CD). As a result we found that the CD was 1.675*10^-6 meters. The accepted value for the slit spacing is 1.6*10^-6 meters which gives us a percent difference of 4.6%. This falls well within our percent uncertainty of 31%.

This experiment allowed us to find a real world application for laser diffraction and how it can be used to measure defects in CD spacing. With more precise instrumentation would could significantly decrease our percent uncertainty and difference to more accurately the CD's spacing. Also, in this experiment we made a small angle approximation in order to use our equation y=[m*(lambda)*x]/a. Uncertainty due to this approximation was unaccounted for but because of the small angles between the central and first maximums this uncertainty could be safely ignored.