Metal expands when heated.
We expect to see different heating effects in the two types of pennies. For the alloy penny, we expect to see straightforward expansion of the metal, in proportion to the thermal expansion coefficient of copper. For the bimetallic penny, however, we expect that the stress on the joint between the two metals will cause a much greater effect. This effect will show up as fringes on our interference hologram. We will first cool the penny with liquid nitrogen and make the initial hologram.