Put a straw in a glass of water and it looks bent. The light from the straw bends as it passes from water to air. That is refraction, the bending of light as it passes between media. The bending happens because light travels at different speeds in different materials. It is slower in water than in air, and slower in glass than in water. When light enters a medium at an angle, the change in speed causes it to change direction. The relationship between the angles is described by Snell's law. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. That constant is called the refractive index.

Refraction is responsible for many optical phenomena. Lenses work by refracting light. A convex lens bends light inward, focusing it. A concave lens bends it outward, spreading it. The human eye refracts light through the cornea and lens to focus it on the retina. Refraction also causes mirages, where light bends through layers of air at different temperatures. It causes the sun to appear above the horizon even when it is below. It is used in fiber optics, where light is guided through glass fibers by total internal reflection. Refraction is one of the most important behaviors of light.

Refraction facts

• Definition. Bending of light between media.
• Cause. Change in speed of light.
• Law. Snell's law relates angles and refractive indices.
• Lenses. Work by refraction.
• Applications. Eyeglasses, cameras, fiber optics, mirages.

Refraction is not the same as reflection. Refraction bends through a medium. Reflection bounces off a surface.