Snell's law

Snell's law relates the angles of incidence and refraction to wave speeds in two adjoining media. For light, it is commonly written using refractive indices as n₁ sin θ₁ = n₂ sin θ₂.

Refraction: The change in a wave's direction as it passes between media with different propagation speeds. Snell's law quantifies the direction change at a boundary.
Wave: A disturbance that propagates through space and time, transferring energy or information. Snell's law applies to wave propagation across a boundary.
Prism: A transparent optical element with flat surfaces that refracts and can disperse light. Snell's law predicts how each wavelength bends at the prism's faces.
Ibn Sahl: A tenth-century mathematician who wrote about the geometry of refraction and designed burning mirrors and lenses. His treatise contains an early formulation equivalent to the sine law of refraction.
Reflection: The return of a wave from a boundary into the medium from which it came. Unlike refraction, reflection stays in the incident medium and follows the equal-angle rule.
Refractive index: A dimensionless measure of how slowly light travels in a medium compared with vacuum. Refractive indices let the speed-based law be written as n₁ sin θ₁ = n₂ sin θ₂.
Wave speed: The distance a wave pattern travels per unit time through a medium. The ratio of speeds in adjoining media determines the refracted angle.
Optical fiber: A flexible transparent fiber that guides light along its length, usually by total internal reflection. Its confinement depends on refraction and the critical angle predicted by the law.
Willebrord Snell: A Dutch mathematician whose work on refraction was recorded in the early seventeenth century. The law bears his name, though his relevant work was published posthumously.
Diffraction: The spreading and interference of waves around obstacles or through openings. Diffraction changes wave direction through wavefront spreading, not Snell's interface relation.

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