Why does a penny at the bottom of a glass of water sometimes look like it’s floating on the surface?
Why does a penny at the bottom of a glass of water sometimes look like it’s floating on the surface? asks a reader.
Tricks of light and water are all around us. Gaze at a goldfish through the curved wall of its bowl: The fish looks both bigger and at a different depth than it appears when we look down through the open top. And if the fish decides to hang out on the bottom of the bowl, you may see a fishy mirage, floating on the surface.
And then there’s the drinking-straw-in-the-glass illusion. Put a straight straw in a glass of water, and voila: Instant bendy straw. The fact that a straw appears bent or broken is caused by something that really is bent: light.
Light moving from thin air into solid glass slows (and the light wave bends) as it passes into this denser material. It bends again when it travels out of glass and into water. Reflecting off objects underwater, the light travels back through the water and glass, re-emerging in the open air. At each junction, it has bent a bit more. By the time it reaches your eyes, the light is seriously off course. So the part of an object that is submerged in water, like a drinking straw, appears to have moved seriously out of place.
In another underwater illusion, a slim straw (or slender goldfish) often looks fat and swollen. Why? Rather like a curved funhouse mirror, the curving side of the drinking glass or fishbowl spreads out the emerging light, magnifying the images of submerged objects.
The floating penny illusion is another watery trick of light. Put a penny at the bottom of an empty glass. Now slowly pour water into the glass. As the water reaches a certain level, an image of the coin will appear on the surface, when viewed from the side. How come? Some of the light rays hitting the penny reflect from the back surface of the glass. As they emerge from the water’s surface, they bend as they enter the air. And you see a ghostly penny floating above the real penny.
But you can easily make the floating penny vanish from view, using your own sleight-of-hand. Put your hand on the far side of the glass, and you should still see the floating penny. Now wet your hand and press it against the back of the glass. The penny on the surface will disappear. How come? According to physicist Jearl Walker, of Cleveland State University, your wet hand is preventing the reflection of the coin’s image from the surface of the glass.
A dry hand doesn’t work as well, because it isn’t really touching the glass at every point on the skin’s surface. But when a film of water fills those gaps between skin and glass, the contact area increases to nearly 100 percent. Light bouncing off the penny is then absorbed rather than reflected, and the floating image abruptly vanishes.
For more refraction fun, visit this website.