Refracting

Refracting, in optics and wave physics, refers to the bending of a wave as it passes from one medium into another with a different propagation speed. For light, this change in speed is caused by a different refractive index of the two media, leading to a change in the wave’s direction. The phenomenon occurs whenever the boundary between media alters the wave’s phase velocity.

The relationship governing refraction is Snell’s law: n1 sin(θ1) = n2 sin(θ2), where n1 and n2 are the refractive indices of the two media and θ1 and θ2 are the angles of incidence and refraction with respect to the boundary’s normal. Since the refractive index is n = c/v, where v is the wave’s speed in the medium and c is the speed in vacuum, refraction reflects both velocity change and directional change. Dispersion, the dependence of n on wavelength, causes different colors to refract by different amounts, producing effects such as prisms separating white light into a spectrum.

Refraction can occur at a flat interface or at curved surfaces. A planar boundary changes the angle of transmission, while curved surfaces, as in lenses, can converge or diverge rays to form images. The strength of a lens is described by its focal length and refractive power, both derived from material indices and surface geometry. When light travels from a denser to a rarer medium, total internal reflection may occur if the incidence angle exceeds a critical value, preventing transmission.

Common examples include objects appearing bent or raised underwater, the apparent displacement of a straw in a glass of water, and atmospheric mirages. Applications span eyeglasses, cameras, telescopes, and fiber-optic communication, where controlled refraction guides and concentrates light. The concept dates to early optical experiments, with Snell’s law formalizing the relationship between angle and refractive indices.