Principle

Thermal radiation can be measured at all surfaces as long as their temperature differs from that of the surrounding. Therefore it applies that the hotter an object is, the more radiation it emits. Also the surface colour influences the behaviour: dark surfaces emit more thermal radiation than light ones. An example for application of this effect is a heat sink which is often coated with a black layer to emit more thermal radiation.

Benefits

• Simple set-up
• Affordable experiment
• For both demonstration and student experiments

Tasks

1. Measurement of the room temperature T₀ (in Kelvin) before starting the experiment.
2. Determination and comparison of the emittance for all four sides of the Leslie cube at a constant high temperature. Therefore, the thermal radiation of a cube filled with boiling water is measured with a Moll-type thermopile.
3. Determination and comparison of the emittance for all four sides of the Leslie cube depending on the temperature.
4. Plotting the thermoelectric voltage V_th as a function of the absolute temperature T or rather T⁴ - T₀⁴ for each side of the cube and validation of Kirchhoff's law of thermal radiation for the used Leslie cube with the collected data.

Learning objectives

• Thermal radiation and emittance
• Kirchhoff's law of thermal radiation
• Leslie's cube
• Black and grey body