The shadow effect is subtle (until you see it!) and easily overlooked in nature. Yet I’ve read accounts from others who have independently chanced upon it, both indoors and out. The beauty is, you can easily replicate the phenomenon.
Take two books (one in each hand) into the sunlight. Standing upright, hold the books at arm’s length, making sure that their shadows appear well away from your own and fall onto a smooth surface like a wall or deck; for the most dramatic effect, the books should be at least 3 feet (1 meter) from the surface. Now bring the two books together until their shadows draw near each other. When you see a narrow gap between the shadows (Image #1), lift one of the books (say the right one) by about 6 to 8 inches (15 to 20 centimeters) (Image #2).
Finally, keeping the vertical offset between the books the same, move them so that their shadows draw closer to each other (Image #3). Watch carefully as the shadows inch closer. The shadow of the book that’s closest to the ground will be the one that appears to leap (Image #4). Reverse the position of the books, and you’ll see the opposite shadow reach out and “grab” the other.
As the light gap narrows between the books, the dominant one’s interference bands appear to move outward as an increasing number of dark bands become visible farther away from the dominant book shadow. As a result, the eye sees an unresolved mass of darkness appear to “move outward” from that book and all but leap toward the other (a grand optical illusion).
Let your fingers do the talking
This diffraction phenomenon is a simple way to demonstrate a peculiar optical phenomenon seen through a telescope. Called the black drop effect, it is an apparent bulging of Mercury’s or Venus’ disk whenever the planet’s silhouette touches the inside limb of the Sun at second or third contact during a transit. So, what some people observe beneath their feet also occurs when the inner planets transit the Sun, such as Mercury will May 9, 2016.
By the way, people who need reading glasses may want to try a similar experiment to see how “bad seeing” can mimic the phenomenon. Repeat the above experiment using your index fingers in front of a computer screen. Place yourself about 20 inches (50cm) from the screen, and hold your fingers upright at half that distance, placing one a bit closer to you than the other. Close one eye, and then gradually move the fingers toward one another without changing their distance from you. A substantial bulging should occur. Try this again with your reading glasses on, and the phenomenon doesn’t occur!
As always, send your observations and thoughts to firstname.lastname@example.org.