We say an object is perfectly black if it absorbs all electromagnetic radiation that lands upon it, regardless of wavelength. As we showed in Radiative Symmetry, a perfect absorber is also a perfect radiator. Black bodies radiate more heat per unit surface area than any other type of body.
Hotter bodies radiate heat using shorter wavelengths. The graph below shows the distribution of radiated power with wavelength for black bodies at 300 K and 3000 K.
The absorption spectrum of water is such that a 20-μm film of water will absorb 95% of the heat radiated by a black body at 300 K, but less than 1% of the heat radiated by a body at 3000 K. Room temperature is roughly 300 K. An incandescent lamp filament reaches roughly 3000 K.
We took a shiny brass disk and let it warm up beneath an incandescent lamp with various treatments of its top surface. In some experiments, we covered it with a film of water. Sometimes we covered the water with a clear plastic sheet. The plastic is transparent to visible light. It may or may not be transparent to radiation from the disk and the lamp. Sometimes we covered the lamp itself with the same type of plastic sheet. For a more detailed description of our apparatus and procedures, see here.
In each experiment, we record the equilibrium temperature of the brass disk. We will present our results in our next post. In the meantime, we invite you to consider our various arrangements and see if you can anticipate our results.