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The image of the slide is focused in the air. The moving wand reflects the image piece by piece. Our retina retain each piece of the image for a fraction of a second - long enough to let we put the pieces together to make up the whole picture.
If we collect the image
"flying" in the air with a round object like a plasitic bowl,
the image appears deformed; if the bowl is semitransparent, we can
see the image also on the back of the bowl.
