Thermal Radiation. Radiate it.

Emission (Leslie cube):

1. Fill a Leslie cube (four different faces: dull black, dull white, shiny black, shiny silver) with boiling water.
2. Place an infrared detector (thermopile) a fixed distance from one face.
3. Record the detector reading; repeat for each face at the same distance and temperature.

Absorption: shine a radiant heater equidistant between a dull black and a shiny silver plate, each with a thermometer or wax-held coin on the back; the black plate warms faster.

Reflection: aim the heater at a surface tilted at an angle and place the detector along the reflected ray. The shiny silver surface reflects most of the infrared (a strong reading); the dull black surface absorbs it instead, so almost nothing reflects. For an opaque surface, reflectivity = 1 − absorptivity, and absorptivity = emissivity (Kirchhoff's law) — so a good absorber is a poor reflector.

Result: dull black is the best emitter and absorber but the worst reflector; shiny silver is the worst emitter/absorber but the best reflector. This is why vacuum flasks and survival blankets are silvered.

• Same distance for every face/plate — radiation intensity falls with distance.
• Same temperature of the cube/source throughout (top up with hot water).
• Same detector orientation — face it squarely at the surface.
• Background radiation — shield the detector from other hot objects and direct sunlight.
• Surface area the same for each face.

• 2.3.3 Radiation — recall that thermal radiation is infrared and requires no medium; describe how surface colour and texture affect emission and absorption; describe experiments to distinguish good and bad emitters/absorbers; relate to applications (e.g. shiny kettles, black solar panels).