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Experiment_11

# Experiment_11 - Experiment 11 Fourier Transforms and...

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Experiment 11 Fourier Transforms and Spatial Filtering Introduction There are four parts to this week’s lab. You will be looking at images of periodic objects. In the last section you will be using Fourier Series to calculate the pattern recorded. Be sure to read the theory paper handed out in class. 1 Crossed Rulings Procedure Place a laser with spatial filter at one end of the optical bench. In front of the laser, place a lens so that you get a beam of parallel light. Just beyond the lens, place two 300 lines/inch Ronchi rulings. Make sure that the rulings are in contact and that they are crossed at an angle of 90 degrees. Thus the two rulings will make an array of square apertures. Place an eyepiece with frosted glass about 120 cm from the rulings. Place a 10 cm lens about 15 cm from the rulings. In order to find the image of the rulings in the eyepiece, place a piece of frosted glass between the collimating lens and the ruling. The glass should be as close to the rulings as possible. Now move the 10 cm lens toward the Ronchi rulings until you can see the magnified image in the eyepiece. Now remove the frosted glass. Observe the image of the Ronchi rulings in the laser light. Next place a variable slit in the focal plane of the 10 cm lens, between the lens and the eyepiece. Make sure that the diffraction pattern is in focus on the variable slit. Also make sure that the center of the slit is in the center of the diffraction pattern. Now close the slit until only the central vertical row of spots in the diffraction pattern passes through the slit. Examine the image in the eyepiece now. Record your observation. Analysis Report you observations. How does the pattern change as you close the slits? Explain, in terms of the Fourier transform, the pattern seen in the eyepiece. Why are there only horizontal lines?

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