Radiation properties of different bodies

So far we have mainly looked at absorption in reference to radiation. However, this is only one aspect in the analysis of radiation.

We only see the optical radiation that reaches our eyes. The eyes receive this radiation and absorb it. The received radiation must therefore either emanate from the surface seen body (emission) or must be scattered on it (reflection).

Kirchhoff's law of radiation:

If bodies are in thermal equilibrium with each other, then they have the same temperature.

If we balance the heat fluxes, the dissipated energy must be equal to the absorbed energy, otherwise a change in temperature would be detectable. For the radiation equilibrium this means:

Emission = absorption

The image shows two plates opposite each other. They both have the same temperature. The opposite sides are insulated so that no heat flux takes place. If we image these plates have a very large surface and a small distance between them, now an exchange of radiation between the two plates is a given.

Plate (A) emits the maximum possible, epsilon = 1. Plate (B) has an epsilon of 0,6. In order that absorption is equal to emission, plate (B) absorbs 60% of the incoming radiation. The remaining 40% is reflected.

In order for there to be equilibrium, the emitted radiation must be equal to the absorbed radiation.

Plate (B) also emits radiation according to its temperature.
The same goes for plate (A). It receives the 60% of emitted radiation from plate (B) plus the 40% which plate (B) reflects.

==> Both plates are in thermal equilibrium because the amount of emitted radiation is equal to the amount of absorbed radiation.

Absorption/reflection/transmission:

The previous example shows that there are other properties besides emission and absorption of radiation. Radiation can be absorbed, emitted and reflected. There is also the property of allowing radiation to pass through. This is evident in gases and window glass, but in these cases the properties are dependent on the wavelength.

The entire amount of radiation acting on a body is absorbed (a), reflected (r) or let through (d):
a + r + d = 1

For better understanding, transmission and reflection have been added to the emission behaviour of window glass.