Diffraction Lab

Follow these directions, write your own answers to the questions in bold.

Part one - Single Slit

  1. Go to https://phet.colorado.edu/en/simulation/wave-interference and run the simulation

  2. Click on "Slits"

  3. Below "Amplitude" on the right side, click the rightmost icon that looks like a flashlight. (We want to look at light diffraction)

  4. Below that, click "Screen" and then "Intensity" (in that order) Don't click "Graph"

  5. You should have Amplitude at "max" and the slit width should be at 500 nm. The frequency should be right in the middle.

  6. Click the green button on the light generator on the left side to start the simulation, and let it run long enough for the intensity graph to settle down. This is a pretty narrow slit, so the diffraction pattern is wide. Click the "+" below the Intensity graph a couple of times to make the graph bigger

  7. Now drag the "Slit Width" control to 1000 nm to make the slit twice as wide.

  8. What happens to the intensity of the pattern?

  9. What happens to the width of the pattern?

  10. What is the relationship between slit width and the width of the central maximum on the screen?

  11. Observe the intensity graph carefully. Notice that there is a subtle minimum to the side of the central maximum. Make a note of where this minimum occurs. (Hitting "+" below the graph helps you see this - do that even though the central maximum goes off the graph)

  12. Keep the slit width at 1000 nm, and now try sliding the frequency to the right to get the highest frequency, and the smallest wavelength. Wait for the intensity graph to settle down.

  13. Try the minimum frequency and largest wavelength.

  14. What is the relationship between the wavelength and the width of the central maximum? (Remember, high frequency is short wavelength)

Part 2 - Double Slit

  1. Go to https://phet.colorado.edu/en/simulation/wave-interference and run the simulation

  2. Click on "Slits"

  3. Below "Amplitude" on the right side, click the rightmost icon that looks like a flashlight. (We want to look at light diffraction)

  4. Below that, click "Screen" and then "Intensity" (in that order) Don't click "Graph"

  5. On the top of the lower box on the right, change the pull down menu to "Two Slits"

  6. You should have Amplitude at "max" and the slit width should be at 500 nm. The frequency should be right in the middle, and the slit separation you should have at 1500 nm

  7. Click the green button on the left side to turn on the light, and wait for the classic two slit pattern to emerge on the intensity graph. Notice that there are minima and maxima on your graph with a central bright spot. Notice that the maxima get fainter as you go out. This overall envelope of decreasing brightness is caused by the single slit diffraction pattern we looked at before, and the underlying smaller pattern of intensity is caused by the two slits interfering with each other just like two sources would.

  8. Mess with the slit width and observe the overall envelope get wider as the slit width decreases, and narrower as the slit width increases. Leave the width at 500 nm when you are done.

  9. Mess around with the slit separation, waiting after each change to let the intensity pattern settle down.

  10. What is the relationship between the slit separation and the distance separating the maxima on the screen?

  11. With the slit separation at 2000 nm, mess around with the frequency, waiting after each change to let the intensity pattern settle down.

  12. What is the relationship between the wavelength and the distance separating the maxima on the screen? (Remember, high frequency is short wavelength)