Why does light bend in water? asks Kamie Meador, of Oreana, IllinoisA drinking straw (or a pencil) in a glass of water. Your legs in a swimming pool. A tree, half-submerged in a swamp. What they all have in common is a trick of light, one in which objects appear bent or broken where they enter water.
The bendy illusion is a clue that something is happening to light when it travels from one medium -- say, air -- into another, like water. Just as we find it easier to walk through air than to wade across a pool, so does light have a tougher trip, too.
A beam of light zips through a vacuum -- empty space, no molecules in its way -- at about 186,000 miles per second. In fact, the speed of light in a vacuum is a constant of our universe, a precise 299,792,458 meters per second. (Scientists call this number "c.")
The weightless particles that make up a beam of light, called photons, slip through the empty space between molecules here on Earth at the same unimaginable speed. But while light can easily pass through a transparent material like air or water, its speed and direction change as it encounters atoms and molecules along the way.
During the run-ins, atoms absorb and re-emit photons, like a game of hot potato.
So as light passes through, it is slowed slightly, and its path bent a bit. Scientists call the bending "refraction." Since the molecules in different materials are packed more or less densely, light's speed and direction change more or less, too.
Glass is denser than water; both are denser than the air in Earth's atmosphere. The result? Light's speed drops abruptly as it travels from air into glass, slowing to about 124,000 miles per second. So during its brief journey through the side of a drinking glass, light loses some of its zippiness. But when it passes into water inside the glass, it speeds up, to about 140,000 miles per second. And when it exits the glass and streams into thin air, it accelerates to near its speed in a vacuum.
When it comes to bending light, scientists compare all materials to a vacuum, which is assigned a refractive index of "1." By contrast, Earth's air has a refractive index of around 1.0003. The refractive index of solid glass is a much higher 1.5, while water's bending index is about 1.33.
By the time it reaches your eyes, a light beam that has traveled from air into glass or water and back out again has changed course several times. So when a submerged straw, swimmer, or tree appears shifted out of place, it's not the object that's bent; it's the path of the light illuminating it.