My daughter would like to know why straws seem to look bent in liquid, writes Laura Zabinski.
Have you ever slowly dog-paddled in a swimming pool on a sunny day, and glanced down at your legs? Like a straw propped in a glass of water, your legs may look bent or broken.
The illusion that makes straight straws look broken in two, and bendy straws extra-bendy, is made possible is by the tricky interaction between water and light. See for yourself: Immerse a straw (or pencil) in a clear glass of water, making sure that part of it remains above the surface. Now look at the straw from different angles. From the right angle, you should see the straw apparently split in two. The submerged part may look swollen, too, as if it’s absorbed some of the water around it.
(View an image of a pencil in liquid here.)
The illusion occurs because light, unlike rigid straws or pencils, really does bend dramatically as it goes from one material into another, such as air into water. This bending is called refraction. How it works: At the surface of each new material it strikes, light suddenly changes speed. Why? Each material is more or less dense than the other.
The molecules in air are spread far apart, and light can zip through air without being slowed down by encountering too many. But the molecules in glass, a solid, are closely packed, slowing light as it makes its way through. And while water is less-dense than glass, it is more dense than air. (Scientists say air has a “refractive index” of around 1.0003, while water’s refractive index is about 1.33. The index of glass ranges from about 1.5 to 1.9.)
These speed bumps also make light change in direction. So as a beam of light moves from thin air into dense glass, it slows and bends a bit. It bends again when it speeds out of glass and into the water. Finally, it hits the straw.
Reflecting off the submerged part of the straw, light bounces back through the water, pushes through the glass, and emerges in the air on its way to your eyes. At each junction, it has bent a bit more. So by the time it reaches your eyes, the light is way off course. Compared to half of the straw above water, whose reflected light has traveled only through air, the submerged half appears way out of place.
Adding to the illusion, a glass’s curving side spreads out the exiting light, magnifying the image an object underwater. Voila: a fat, broken straw.
Although we don’t notice the difference, the amount light is refracted actually depends on its wavelength, or color. In glass, light’s speed varies by about 2 percent between each color of light (red, orange, yellow, green, blue, indigo, or violet, the rainbow spectrum hidden in white light.) So the index of refraction is usually set for green, an average which works well for a beam of sunlight passing through a glass on your kitchen counter.
