Design Nomenclature / Light & Optics

Refraction

Also called Snell's law, bending

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.

How this term connects

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