What is meant by 'deviation' in optics?

The term 'deviation' in optics refers to the change in direction of light rays, which can occur when light passes through different media, such as air into glass or when it is refracted by a prism. This change in direction is essential for understanding various optical phenomena, including how lenses and prisms manipulate light to create focused images or disperse colors.

When light enters a medium with a different refractive index, it slows down and bends, resulting in a deviation from its original path. This principle is fundamental in designing optical devices like eyeglasses, cameras, and telescopes, as it allows control over how light is directed and manipulated to achieve the desired visual results.