Courses: P4, P14
A laser with a spacial filter illuminates a ball bearing. Counter-intuitively, the image against a screen at some distance shows a bright spot in the center of the ball bearing -- while most people would expect the laser to be completely blocked by the ball bearing's shadow. One can also see interference patterns around the edge of the shadow of the bearing.
Setup: This setup is stored largely ready to go. The laser has a spacial filter on it that produces a nice wide beam. Place the ~1 cm ball bearing, cemented to a glass slide, in the beam. Adjust the aim of the laser so the ball is centered in the beam -- this can be more precisely adjusted by observing the diffraction pattern on the screen/wall. Adjust until the pattern is close to round. Poisson's spot should be clearly visible in the center of the ball bearing's shadow. It doesn't make a huge difference how far the ball bearing is from the laser -- but I believe closer is slightly better. It will be best to project this onto a wall quite a distance from the laser in order to enlarge the image somewhat.
DON'T mess with the spacial filter adjustments! It's fairly tedious and time consuming to get the filter back to where it belongs, so the hope is to keep it properly adjusted and never change it. That said, it does periodically get out of adjustment. Mount a piece of white paper a few inches in front of the laser. Try adjusting the spacial filter back and forth several turns in each direction in one axis, then move the other axis a small amount and try again. Repeat this over and over until you find the spot where the image becomes significantly brighter, then iterate around that spot. When correctly adjusted, the spacial spot should be very bright and uniform.
