Negative Pinhole
Safety Warnings
Warning: Hot Lamp Surface
Purpose
Demonstrates the "negative pinhole" effect: small opaque objects cast shadows that transition from object-shaped to source-shaped as the object moves closer to the light. Students learn that rectilinear propagation allows the gaps around a small obstacle to act as imaging apertures that project the lamp filament's shape.
Figure 1:
Two pith balls suspended from a bent rod, positioned near the lamp. The shadows on the wall show the filament shape rather than the ball shape, demonstrating the negative pinhole effect.
Figure 2:
Close-up of a single pith ball near the lamp. The shadow on the wall takes the shape of the lamp filament rather than the ball.