Blackbody Boxes - Physics Demonstrations - Blackbody Boxes | Physics Undergraduate Labs

Purpose

Two boxes with different interior colors (white vs. black) show identical black apertures when closed, demonstrating that a cavity with a small opening approximates an ideal blackbody regardless of interior surface properties. Students learn that blackbody behavior depends on geometry and multiple internal reflections, not surface color.

Two cardboard boxes of different sizes with dark paper roofs and white block "chimneys," each with a black paper panel on the front containing a circular aperture that appears identically black

Same two boxes with lids opened; the larger box on the left now shows a bright white hole revealing its white-lined interior, while the smaller box on the right retains its dark aperture

Figure 1: Both boxes show identical black apertures when closed, despite having different interior surface colors (white vs. black lining).

Figure 2: With lids open, the larger (white-lined) box's aperture turns white while the smaller (black-lined) box stays dark, proving that cavity geometry — not surface color — caused both to appear black when closed.