all things pertaining to light — how it bends and reflects,
how it splits into colour, and what it is when you look closely
Rays, lenses & mirrors
A steerable optical light-bench — drop emitters, mirrors, lenses, prisms and blocks onto a dark bench and drag them around; beams re-trace live by the real laws of geometric optics (reflection, Snell's law with total internal reflection, an ideal thin lens, Cauchy dispersion that fans white light into a true rainbow). It keeps the promise: a self-test proves the optics exact — Snell to a billionth, the critical angle dead on, the focus where the math says. Open ▸
How a far thing is made to look near — a telescope optical-diagram bench. A Keplerian refractor (objective + eyepiece) forms a real inverted image and relays it to the eye magnified M = f_obj/f_eye; a Newtonian reflector folds a parabola's perfect focus out to the side. Drag the focal lengths and watch the rays and magnification update. It keeps the promise: a self-test proves M is exact and the parabola focuses a parallel bundle to a single point — no spherical aberration. Open ▸
The lighthouse's great innovation: collapse a thick converging lens into concentric rings of prism and it keeps the focus but loses the bulk — light enough to float, bright enough to throw a beam for miles. Watch a lens flatten into a flat Fresnel sheet whose every ring still bends light to the same focus (each facet angle solved by exact Snell's law), then put a lamp at that focus and the rings collimate it into a sweeping beam. It keeps the promise: a self-test proves every zone focuses a parallel bundle to a single point — to machine precision. Open ▸
The oldest optical instrument — and an image with no lens at all. A pinhole in a dark room's wall casts the world outside, upside-down and reversed, on the far wall. Shrink the hole and the picture sharpens… until it doesn't: too small, and diffraction blurs it back. Watch the geometric and diffraction blurs trade off and meet at the optimal pinhole, where the image is crispest. It keeps the promise: a self-test proves the magnification is exactly v/u and that the best hole size lands where the two blurs cross. Open ▸
A tumbling chamber of translucent glass seen through an N-fold mirror — turn the symmetry order from 3 to 12 and the shards fold into a living rosette that holds perfect dihedral symmetry every frame. Reseed the glass, change the skin, freeze and export. It keeps the promise: a self-test proves the image is exactly Dₙ-symmetric — every rotation and reflection, to machine precision — a true mirror, not a fake rotate. Open ▸
A smear of nonsense on the table that a mirror puts right — pre-distort a picture so a vertical cylindrical mirror at its centre reflects it back into the true, undistorted image. The old conjurer's trick, by the exact reflection geometry: see the source, its warped table-print, and the reconstruction the mirror would show, side by side. It keeps the promise: a self-test proves the warp and its inverse round-trip to machine precision. Open ▸
Colour & spectrum
The rainbow, derived from one raindrop — trace sunlight refracting in, reflecting inside, and refracting out of a spherical droplet, and the rays pile up at the angle of minimum deviation into a bright bow. Watch the primary at ~42° and the reversed secondary at ~51° form, with Alexander's dark band between and the colours spread by the water's dispersion. It keeps the promise: a self-test proves the bow angles fall out of Snell's law alone, not from baked-in numbers. Open ▸
The rainbow's frozen twin — where water drops give rainbows, hexagonal ice crystals high in the air give halos. Sunlight refracting through their 60° faces piles up at minimum deviation into the great 22° halo around the sun; their 90° faces give the fainter 46° halo, drifting plate crystals throw bright sundogs to either side, and a jewel-like circumzenithal arc hangs overhead. It keeps the promise: a self-test proves every ring's radius falls out of Snell's law and the ice's index — the 22° halo at 21.8°, nothing baked. Open ▸
Split light and read what it's made of — disperse a beam through a prism (Cauchy dispersion) or a diffraction grating (d·sinθ = mλ), then switch the source to see real line spectra at their true wavelengths and colours: hydrogen's Balmer lines, sodium's yellow doublet, mercury and neon, and the Sun's dark Fraunhofer lines. It keeps the promise: a self-test proves the Balmer lines fall out of the Rydberg formula and every order lands where the grating equation says. Open ▸
The colours that come from nothing but thickness — a soap bubble, oil on a puddle, Newton's rings. Light reflecting off the two faces of a film thinner than a wavelength interferes with itself, cancelling some colours and reinforcing others, so the hue is the whole spectrum minus what the path-difference erased. Every colour here is computed from the real interference and integrated against the eye's own colour-matching curves — not a painted gradient. It keeps the promise: a self-test proves the rings fall where 2t = mλ and that a flat spectrum integrates to neutral. Open ▸
The wave nature of light
Light has an orientation — and two filters can prove it. Send unpolarised light through a polariser and an analyser and rotate them: the brightness follows Malus's law, I = I₀cos²θ, from full to pitch-dark when crossed. Then slip a third filter between the crossed pair and watch light reappear where there was none — the three-polariser paradox. It keeps the promise: a self-test proves the transmitted intensity equals cos²θ to a billionth, and crossed is exactly zero. Open ▸
An opening doesn't cast a sharp shadow — light fans out. In the far field the pattern is the squared Fourier transform of the aperture: one slit gives a sinc, a row of N slits — a grating — multiplies it by a sharp comb whose bright orders obey d·sinθ = mλ. Raise N and the orders sharpen (why a grating out-resolves two slits); switch to white light and each order fans into a spectrum — a grating is a spectrometer. It keeps the promise: a self-test proves the rendered curve equals a direct Fourier transform of the very slits drawn above, to a part in a million. Open ▸
A Morpho butterfly's electric blue holds no blue pigment — its colour is pure structure. Stack a few dozen transparent layers, alternating high and low refractive index, and every interface reflects a faint echo; at the right wavelength they all add in phase and the stack becomes a mirror for a whole band of colour — a photonic band gap. Tilt it and the band slides bluer, the fingerprint of colour made from geometry, not dye. It keeps the promise: the reflectance is computed by the full transfer matrix, while an independent Bloch band theory predicts exactly where the gap must be — and a self-test proves they agree. Open ▸
Light is one of many things that ripple. Its wave-cousin lives on the workbench: Ripple, a wave-interference tank where you can watch the same superposition that makes the colours above play out, slowed down, on dark water.