I'm sorry this reply is so late, but things got away from me over the
The light source should shine on the face of the face. Try putting the two
pieces of plastic (one on top of the other) on top of a piece of black paper
on a flat table top, under a table lamp or other light source. Then press
hard with your thumb, and look at the area around your thumb to see the
colored fringes. Don't look straight down, but instead look at an angle so
that your thumb seems to be right in the reflected image of the light bulb.
Hope this helps.
Exploratorium Teacher Institute
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