Holographic interferometry

The arrangement for this hologram is similar to that shown in in the two beam holography experiment, except for the addition of a second lens to enlarge the object beam more. Of course the object is different too. It is now a softdrink can painted white. Gently slip a rubberband around the waist of the can. This will make a slight depression in the can where the band rests.

Block the beam from the laser as before, and load a glass plate into the holder. It is essential that the plate be oriented with its emulsion side toward the laser light. The box will be marked with the orientation of the plate. Take care to preserve this orientation when you put the plate in its holder, because unlike the film there is no notch to guide you. You can check the orientation if you're curious. Photograph emulsion is sticky when damp, so you nibble an edge of the plate and note which side sticks to you!

Now make a 3 second exposure. Be very careful not to disturb anything when you return the card to block the laser again. Use a sharp razor blade and nick the rubber band. It will fly off without moving the can. Take a second exposure for the same length of time.

Process the plate, bleach and dry it. After it has dried view it with the second laser. You'll see a brilliant image of the can, but it now will have several interference fringes on it. Each fringe represents a successive displacement of the can's surface by $\lambda/2$, or $3.16\times10^{-5} cm$.

How far did the band deflect the center of the can? Why is there a fringe for each half wavelength?