Prism

Written for the variant you picked above: Night

Build a canvas 2D background of light through a prism, computed rather than drawn. Scene: a dark wall falling into a floor (a hard gradient stop at 70%), an equilateral prism (side min(30% W, 50% H)) slightly right of centre, clear glass: a faint gradient fill, a hairline edge and a brighter highlight on the left face, and a faint reflection on the floor. Optics: the light source sits low on the left (breathing slightly) or follows the pointer, clamped to the left of the prism. The white ray aims at a point just above the middle of the left face. For 72 wavelengths from 400 to 700nm compute n(λ) with Cauchy's equation for crown glass (A 1.5046, B 0.0042 µm², the B term scaled ×7 so the fan reads at screen scale), refract at the left face with Snell's law in vector form (t = ηd + (η cos i − √k) n; null on total internal reflection), follow the inner ray to the face it reaches and refract into air — or, past the critical angle, reflect internally and continue (up to three bounces), so the light always leaves somewhere, as in real glass. Drawing (additive "lighter" compositing): the white beam as four widening translucent quads that fade in from the source; a faint fan inside the glass; then for each wavelength a wedge from its exit point to far beyond the canvas, spanning halfway to its neighbours' far points, so adjacent colours overlap and blend into a continuous spectrum — once spanning ±2.3 neighbours and once ±4.2, faint, for bloom. Wavelength → RGB uses the standard piecewise approximation with intensity falling at the ends. A soft hot spot where the beam enters. ~160 dust motes drift; each is drawn only when inside the white beam or inside the fan's angular range, in the colour there. One rAF loop while visible (~33fps on touch); the source eases toward its target. Reduced motion: one still frame. Selected variant — Night: theme="night": a near-black wall and floor, so the spectrum glows at full contrast.

Notes

Interaction

Move the pointer on the left to move the light; the spectrum swings and spreads accordingly.

Motion

Source eases at 6/s; the resting light breathes on a ~18s cycle; dust drifts.

Accessibility

The canvas is aria-hidden and decorative; overlay content sits above it. Reduced motion draws one still frame.

Responsive

The prism is sized from the canvas and centred on narrow screens; everything is recomputed on resize.

On touch

Drag a finger on the left to move the light; it drifts back when you let go. ~33fps.