I don't know what the generic name is, but this illusion is very common. Any
toy store or toy department would probably contain several examples. As to
how they work, the card consists of a plastic coating over a printed
background. The coating is covered with many parallel ridges that act
somewhat like prisms. The two (or more) images that you see (e.g. ball not
thrown, ball thrown)are both on the printed background. Each image is cut
into narrow stripes. The stripes of one image are interleaved with the
stripes of the other image. When viewed through the ridges of the coating,
you can see one image or the other, depending on the tilt angle of the card.
Four to five images seems to be the practical maximum. Educational
Innovations carries a nifty little demo called a "cine-spinner" that uses
this principle, except that the cine-spinners accomplish the illusion with
thick plastic, opaque stripes, and a bit of parallax.
Vickie Frohne
-----Original Message-----
From: Tony Wayne [mailto:wayne@PEN.K12.VA.US]
Sent: Thursday, June 06, 2002 7:55 AM
To: PHYS-L@lists.nau.edu
Subject: Optical toy
I have a old mini baseball card (from cracker jack) where the pitcher
switches positions when you tilt the card at different angles.
(1)Does anyone know what these cards are called?
(2) Does anyone know how they are made/work?
-tony
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