Disco Ball Light Rays: 2D Specular Reflection Optics

Ball

Facet count 48

Rotation speed 30°/s

Light source

Distance from ball 4.0×R

Starting angle 0°

Orbit speed 18°/s

Live Readout

Ball angle

0.0°

Light angle

0.0°

Illuminated facets

Wall spots this frame

How it works

Top-down 2D cross-section of the disco ball: N flat mirror facets are placed evenly around a circle of radius R, each with an outward unit normal n = (cosθ, sinθ) where θ is the facet's angular position plus the ball's current rotation. A single point light source sits at a controllable distance and orbit angle around the ball.

d = normalize(facetPos − lightPos)   // incident direction
lit = dot(n, d) < 0                  // facet faces the light
r = d − 2·dot(d, n)·n                // law of reflection

For every lit facet, the reflected ray r is traced from the facet position outward until it intersects one of the four room walls (a standard ray/rectangle slab test), producing a light spot there. As the ball rotates and the light orbits, each facet's normal sweeps through a different angle every frame, so the reflected direction — and therefore the spot's position on the wall — is recomputed from the real geometry, not scripted.

Bright ticks on the ball — facets currently facing the light (dot(n,d) < 0).
Thin colored lines — the traced reflected ray from each lit facet to its wall hit.
Fading glow trail — the canvas is not fully cleared each frame, so the last second or so of spot positions persists, revealing the moving pattern the rotating facets sweep across the walls.

Disco ball facets Light source Reflected ray / wall spot