Cavity radiation

The classical theory an oven, with inside walls at high temperature. The radiation emitted from the inside surfaces would bounce around the cavity, and would sometimes be reflected and sometimes absorbed by the walls. Eventually a state of equilibrium would be reached, when the energy emitted by the walls equalled the energy absorbed. The radiation inside the cavity could then be defined as the equilibrium blackbody radiation corresponding to the oven temperature. It does not depend on the size or shape of the cavity, nor on the material of its walls.

of blackbody radiation used what may at first appear to be an unlikely model, consisting of an enclosed cavity, like

We can visualise the radiation criss-crossing the cavity in all directions carrying energy, as illustrated in Figure 11.3. At equilibrium there is no net flow of energy to anywhere, and the flux across any area element inside the cavity is the same in all directions, and balances out. Nevertheless, there is continuous traffic of energy in such an isothermal enclosure and we can express this in terms of a quantity, y, the radiant energy density at a point.

There is a small opening in the wall of the cavity. The idea is that any radiation which enters has practically no chance of finding its way out. As a result the entrance to the cavity becomes a perfect absorber. Much more important, it is also a perfect emitter, or a window through which a sample of the radiation leaves the cavity.

Figure 11.3 Electromagnetic waves are bouncing around inside the cavity, reflecting off the walls. Radiant energy per unit volume in the wavelength range A, X + dA, is y/XdX.

Figure 11.4 Radiation emerging from the cavity looks light from burning coal.

Small opening

Figure 11.4 Radiation emerging from the cavity looks light from burning coal.

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