The Kirchhoff's law leads to the conclusion that the good radiators of thermal radiations are

Explanation

Kirchhoff's law of radiation : At a given temperature, the coefficient of absorption of a body is equal to its coefficient of emissoin.
Theoretical proof : Kirchhoff's law can be proved with the help of the following thought experiment.
Suppose that an ordinary body A and A blackbdoy B are enclosed in an athermanous enclosure as shown in the figure.

According to Prevost's theory of heat exchanges, there is a continuous exchange of heat between each body and its surroundings Hence, the two bodies, after some tine, will attain the same temperature as that of the enclosure. ltbr. Let a and e be respectively the coefficients of absorption and emission of body A. Let E and `E_(0)` be the emissive powers of bodies A and B, respectively. ltbr. Suppose that Q is the quantity of radiant energy incident on each body per unit time per unit surface area of the body.
Body A will absorb the quantity aQ per unit time per unit surface area and radiate the quantity E per unit time per unit surface area. Since there is no change in its temperature, we must have ltbrrgt `aQ=E" "`.........(1)
As body B is a blackbody, it will absorb the quantity Q per unit time per unit surface area and radiate the quantity `E_(b)` per unit time per unit surface area.
Since there is no change in its temperature, we must have
`Q=E_(b)" "`.......(2)
From Eqs. (1) and (2), we get,
`a=(E )/(Q)=(E )/(E_(b))" "`........(3)
By definition of coefficient of emission,
`e=(E )/(E_(b))" "`...........(4)
From Eqs. (3) and (4), we get, a=e
Hence, the proof of Kirchhoff's law.