Colored Shadow Illusion - Physics Demonstrations - Colored Shadow Illusion | Physics Undergraduate Labs

Purpose

Demonstrates that color perception involves neural processing, not just physics. Superimposed white and colored lights create two shadows: one in the color of the colored source, and one that appears to be the complementary color despite receiving only white light. This is direct evidence for the opponent processing theory of color vision.

Setup with LED spotlight on wooden platform at left, circular mask on ring stand at center, optical cable light source at right, and projection screen showing a cyan shadow (left), yellow shadow (right), and dark overlap where both lights are blocked

Montage of three horizontal strips with preset white balance: top strip shows red colored shadow on the left and a gray/neutral shadow on the right; middle strip shows green/cyan colored shadow left, gray right; bottom strip shows blue colored shadow left, faint purple right

Montage of three horizontal strips with auto white balance: top strip shows red shadow left, cyan-tinted shadow right; middle strip shows green shadow left, pinkish/magenta shadow right; bottom strip shows blue shadow left, yellow/amber shadow right

Figure 1: Complete setup: LED spotlight (left), circular mask on ring stand (center), optical cable white light source (right). The screen shows a cyan shadow from blocking the white light, a yellow shadow from blocking the colored light, and a dark region where both are blocked.

Figure 2: Montage with camera white balance preset to match the white light source. The colored shadows (left) appear vivid, but the white-light shadows (right) look neutral/gray — close to the physical reality of the light reaching them.

Figure 3: Same shadows with camera auto white balance (mimicking human vision). The white-light shadows now appear in complementary colors — cyan, magenta, and yellow — just as the eye perceives them in person.

Circular Barrier

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