The phenomenon where light bends as it passes through different mediums is known as refraction. Refraction occurs due to changes in the speed of light as it moves between materials with varying optical densities, such as air, water, or glass. This bending of light is responsible for various optical effects, including the apparent bending of a straw in a glass of water and the formation of rainbows. Refraction is governed by Snell's law and is a fundamental principle utilized in lenses, prisms, and optical instruments.