The assumption that the plates are "very flat" isn't a very good one. Nonetheless, if the plates are very clean it's possible to squish out all the air and see very few fringes. Sometimes, different plate surfaces need to be tried. Here's photo of two sort-of flat plates- remember that the dark fringes represent a full wavelength path length difference. The photo below is of two glass plates pressed together. (They weren't cleaned first.)
This photo was taken with illumination provided by a green inspection lamp (from Edmund Optics). The fringes were visible using a laser reflected from a wax paper covered mirror, but it was too difficult to photograph. (Not enough hands)
The air wedge is made by placing a thin shim (plastic, hair, etc) at one end and pressing the plates gently together. Fringes are generally parallel across the plate. Because the plates aren't very flat to begin with, they aren't exactly parallel, nor are they evenly spaced along the length of the plates. The photo below is a portion of the plates with one end (to the left) held apart by a piece of trash bag (labeled 21.5 micron thickness on the carton).
To take this photo, the plates were on a black binder, which tilted downward toward the right. This makes the fringes look even more unevenly spaced than they were. Fringes were visible along the length of the plates, but those on the left hand side were low contrast and are not visible in the photo.
Since the number of fringes for this wedege was not impossible large, I counted them all- 104 fringes. The thickness of the plastic is given by
When a hair was used to form the wedge the fringes were much closer together, and there were too many (for me) to count. I measured the number of fringes in 1 cm blocks at several locations along the plates, found the average number of fringes per cm, and multiplied by the length of the plates. (The paper under the plate can be marked in 1 cm increments.) There were about 250 fringes total. This gives a hair diameter of
Certainly students can see the effect of the plates not being flat and they can see how opening the wedge angle (using a wider shim) makes the fringes closer together. With two different colors, it's pretty clear that the fringe spacing changes. But the validity of the measurement of shim thickness would be an interesting area of discussion.
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