Lab 5 Fringes - Cell Phone Photos

Here are 3 sets of fringes from the Lab 5 microscope slide experiment, one from each team's successful layout. An incident beam I reflects first off the front surface of the slide creating one reflected beam, and then off the back surface, creating another. These travel almost in the same direction, but not quite, due to the small tilt of surface S1 with respect to S2. This tilt is there because the slide surfaces are not perfectly parallel. The number of fringes and the size of the illumination spot on the slide measure the tilt angle, typically a few mrad.

Reflection geometry. Each reflection is about 4% as intense as incoming beam I. Not shown is the transmitted beam T that continues downward to the left.



The fringe orientation is due to the wedge orientation. In lab we rotated a slide to see that the "clock" angle of the fringes is tied to the slide, not to the rest of the optics.

We also tested what would happen if we overlapped the fringe pattern from one experiment bay on top of the beam from another experiment bay. Additional fringes or no additional fringes? At first sight we didn't see evidence for additional fringing - the pattern looked like the straight addition of the 2 fringe patterns rather than the interference of those 2 fringe patterns. Remember that the number of fringes is related to the angle between the two beams, with about 5 fringes per mrad. The two beams were incident at an angle of several hundred mrad, so there were of order a thousand of these new fringes across the pattern -- too many to resolve since there's more than one fringe per camera pixel. So we didn't see much new. If we had inserted some clever optics to overlay the two beams with a small, mrad sized, angle between them (like is done when you use 2 stacked slides), then what would we have seen at the detector/screen?