Shadows - Physics Demonstrations - Shadows | Physics Undergraduate Labs

Incandescent bulbs become very hot during operation. Allow adequate cooling time before handling bulbs or wrapping with aluminum foil. Use caution when wrapping foil around the tall bulb—wrap while cool, then turn on.

Purpose

Students observe how source geometry determines shadow sharpness. A point source (Mini Maglite) casts sharp shadows; an extended source (frosted bulb) produces penumbra. A tall clear filament bulb shows orientation-dependent shadow sharpness, and wrapping it in foil with a pinhole restores point-source behavior.

Three-panel image: (left) Mini Maglite with wooden bar casting a sharp shadow on the wall, (center) round frosted bulb with the same bar casting a diffuse shadow, (right) tall clear filament bulb with the bar held horizontally

Four-panel image: (top left) tall clear filament bulb with bar held vertically casting a shadow, (top right) same bulb wrapped in aluminum foil with bar vertical, (bottom left) bar held horizontally with unwrapped bulb, (bottom right) foil-wrapped bulb with bar horizontal showing sharp shadow from pinhole

Figure 1: Three light sources compared. Left: Mini Maglite (point source) produces a sharp shadow. Center: round frosted bulb (extended source) produces a diffuse shadow. Right: tall clear filament bulb.

Figure 2: Tall filament bulb with and without aluminum foil pinhole. The unwrapped bulb casts orientation-dependent shadows; the foil pinhole converts it to an effective point source, producing sharp shadows in all orientations.

0.75 × 1.5 × 12 in Wood Block

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Tin Foil Cylinder

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Safety Warnings

Warning: Heat Warning

Purpose