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Flying circus of physics

Can you hide your shadow?

Thursday, August 01, 2013


Can you hide your shadow?
Jearl Walker
August 2013   Several years ago Roberto Casati (of the Institut Jean Nicod in Paris) raised a fascinating question: Can you hide your shadow as a young child might try when placing some object over the shadow? We as adults know that is impossible. If you place, say, a sheet of paper over a shadow on the ground, then the shadow is on the paper. It certainly has not disappeared.

Nevertheless, Casati discovered a way to hide a shadow. In following his lead, here are some of my photographs of a pencil supported in bright sunlight by a cardboard tube. The shadows of the tube and pencil lie across paving stones in my driveway. In these first two photos, I see what I would expect: a merger of pencil and tube shadows.

In this next photo, I covered the tube shadow with white sand, taking some care to keep the sand within the shadow. What I now see is pencil shadow then seemingly crosses under the sandy region. The pencil shadow has disappeared in that region, hidden by the sand.

Of course, this is an illusion created by my visual interpretation of the scene. Although the sand is in the shadows of the tube and pencil, it is still bright relative to the dark pencil shadow on either side because the sand grains scatter some of the ambient light from the surroundings to me. My brain no longer interprets the sandy region as being a shadow region. To make sense of the pencil shadow, which I know must be there somewhere, I interpret it as extending under the sand. I believe the illusion weakens if the two ends of the pencil shadow are too far apart.


Here are links to the photos in Casati’s paper in Perception. In one photo you see a similar hiding of a rod shadow by couscous. In the other you see you see a shadow of Casati himself being hidden by a layer of ice in the overlapping shadow of a playground slide. It was taken early in the morning when the Sun had melted away ice from the rest of the playground but not in the shadow of the slide.

Here is another photo I took following a Casati idea in another of his publications. The pencil, which lies across folded cardboard, creates two visually unrelated shadows, one on the cement walkway and one on the cardboard. Here again I have impression that the connection of the shadows on the left and right are hidden, this time by the cardboard.

Of course, if I invert the cardboard, I see the shadow with no illusion: it just runs up and over the cardboard.

Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
· Casati, R., “How I managed to hide my shadow,” Perception, 36, 1849-1851 (2007)
· Casati, R., “Some varieties of shadow illusions: Split shadows, occluded shadows, stolen shadows, and shadows of shadows,” Perception, 41, 357-360 (2012)

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