Refraction is the bending of light as it passes through different materials. It occurs because light travels at different speeds in different materials. When light travels from one medium to another, it changes direction. This is because the light waves are being slowed down or speeded up as they pass through the new medium. Opticals are devices that use refraction to manipulate light in various ways. Some examples of optical devices include lenses, prisms, and mirrors. Lenses are used to focus or spread out light, and can be made from a variety of materials such as glass, plastic, or crystal. Prisms are used to separate light into its different colors, and can also be used to bend light. Mirrors are used to reflect light, and are often used in telescopes and other optical instruments.