Design Nomenclature / Light & Optics

Refraction

Also called Snell's law, bending

This entry has not been translated yet and is shown in English.

The bending of light as it crosses between media of differing index, because its phase velocity changes; Snell's law fixes the new angle from the ratio of the two indices. It is what makes a submerged rod look broken and what gives a lens any power at all.

Ray diagram at a flat air-water boundary: an incoming ray meets the surface at thirty-five degrees and the transmitted ray continues into the water at twenty-five and a half, bent toward the normal, both angles arced against a dashed normal line and Snell's equation for them printed beneath — with a fainter partially-reflected ray leaving on the incident side at the incident angle.

Not to be confused with

DispersionThe dependence of refractive index on wavelength, so a single refraction sends each wavelength off at a slightly different angle. It is what a prism does, what builds a rainbow, and — inside a lens — what chromatic aberration is: the same image formed at slightly different scales per colour.
Total internal reflectionBeyond a critical angle, light travelling from a denser medium toward a less dense one stops refracting out and reflects entirely back inside. It is why the underside of a water surface reads as a mirror, why fibre optics work, and why a cut gem returns light instead of leaking it.
DiffractionThe spreading and interference of light as it passes an edge or aperture — a wave effect rather than a ray one. It sets the resolution floor of any optical system, puts the star points on a stopped-down aperture, and produces the rainbow sheen on a disc's pitch.

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