Refraction

Refraction is the change in a wave’s direction as it crosses between materials where it travels at different speeds. The change follows from the wavefront entering the new material at different times.

Snell's law: An equation relating a wave’s angles of incidence and refraction to its propagation speeds or refractive indices. It quantifies the bending that occurs when a wave crosses an interface.
Lens: An optical element that refracts light to converge or diverge rays. Its curved interfaces use refraction to form images or redirect light.
Wavefront: A surface connecting points on a wave that share the same phase. Its changing orientation across an interface produces the observed change in propagation direction.
Reflection: The return of a wave from a boundary into the medium from which it came. Unlike refraction, reflection sends energy back rather than transmitting it into the next medium.
Refractive index: A dimensionless ratio describing how much slower light travels in a material than in vacuum. Differences in this quantity predict how strongly light changes direction at a boundary.
Prism: A transparent optical element with flat faces that refracts light and can separate wavelengths. Refraction at its faces redirects light and often spreads it into a spectrum.
Angle of incidence: The angle between an incoming ray and the normal to a surface at the point of contact. It is the incoming geometric quantity used to determine the refracted angle.
Diffraction: The spreading and bending of waves around obstacles or through openings. It redirects waves through spatial spreading, rather than a speed change across materials.
Huygens–Fresnel principle: A principle modeling each point on a wavefront as a source of secondary wavelets. It explains bending by tracking how wavelets advance at different speeds across an interface.
Atmospheric refraction: The bending of light through air layers with varying density and refractive index. It shifts apparent positions of celestial objects and produces effects near the horizon.

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