An executable idealization of additive RGB lighting. The display is calculated in the browser, not a photograph or a measurement. Adapted as an educational experiment from established colored-shadow demonstrations.
An experiment for a white wall
The shadow is made of the other lamps
Three lamps illuminate the same screen. Put an opaque object in the way of the red lamp and the shadow becomes cyan: blue and green are still reaching that part of the wall.
Three rays, a shared obstruction and a receiving surface.
Working model / 2026
Three lamps, one interruption
The apparatus below uses point sources and a circular obstruction. You can switch each light off and move the object laterally. The computer calculates which rays reach each pixel.
Calculated additive-color shadows on a white screen. Ray tests assume ideal opaque circular obstruction and three fixed point sources. Light intensity is normalized; penumbra and spectral variation are omitted.
Field note / The apparent paradox
The color that remains
At each screen point, red, green and blue contributions are added. A shadow blocks one or more contributions along the ray from lamp to screen. If red cannot reach a point, the other two lamps make it cyan. Where red and green are blocked, blue remains. The darkest region is where all active sources are blocked.
Paint works differently: adding colored pigments often removes more reflected wavelengths. This experiment deals with emitted light arriving at the wall.
A real wall, finite-size lamps and imperfect spectra produce softer boundaries and color mixtures that this simple model does not represent.
Three checks with the apparatus
Leave two lamps on
Turn off blue. The illuminated screen becomes yellow where red and green overlap. An obstruction can leave a patch that receives only one of those colors.
Block a single primary
Turn all three on. Find the cyan region: red is missing there. The color belongs to the unblocked light.
Find the dark overlap
Move the object. The narrow region receiving none of the three rays is black. It exists only where the individual shadows overlap.