Question

A perfectly black body is one where:
A. Absorptive power is infinity
B. Absorption point is 0
C. Emissive power is 1
D. Absorptive power is 1

Answer 1

We need to know about this answer, first of all, we learn about blackbody radiation, which is defined as the black body, which absorbs the entire incident light and does not reflect anyone. A black body is also called because it absorbs radiation in all frequencies and emits black body radiation.

Complete step-by-step answer:
We all know that a perfectly black body is one which neither transmits or reflects any radiation; therefore, the absorptance of the body is unity. A black body reflects no wavelength, so; it appears black whenever any color incident on it.
Absorptive power is defined as the ratio of energy absorbed to the ratio of energy incident on a body. Therefore, we can express it as $a = \dfrac{{{E_1}}}{E}$
We must know that here $a$ is the absorptive power, ${E_1}$ is the amount of energy absorbed, and $E$ is the amount of energy incident. We know that for a perfectly black body, the energy absorbed is equal to the energy incident.
$\Rightarrow {E_1} = E$
So we can see that the absorptive power becomes,
$a = \dfrac{{{E_1}}}{E}$…… (I)
We can substitute ${E_1} = E$ in equation (I) and we get,

$$a = \dfrac{E}{E} \\\ \Rightarrow a = 1 \\\$$

Thus the correct answer is absorptive (D).
So we can say that option (C) is wrong since the Emissive power can’t be 1 as we all know that the emissive power is the amount of thermal radiation emitted at a given temperature at all directions per unit area per unit time. So if a body is at 0 degrees centigrade that is 273 K, the emissive power is generally greater than unity.
We can notice that in option (A), the absorptive power is given as infinity which is truly incorrect as we can conclude from equation (I) that the absorbed energy cannot be greater than the incident energy.
We can say that in option (B), the absorption can’t be zero and hence this option is wrong as for a black body the complete incident radiation is absorbed.

Additional information:
Whenever an entirely black body is heated at high temperature, it emits radiation of all possible wavelengths. A blackbody is a physical material with zero reflectivity and transitivity. Blackbody sources are uniform. It depends on the factors, i.e., Temperature and activation energy. Black bodies in thermal equilibrium emit electromagnetic radiations in accordance with Planck's law.

Note: We can say that the difference between the absorptive power and the emissive power is that the absorptive power is a type of ratio of the energy absorbed to the energy incident whereas emissive power is the amount of thermal radiation emitted at a particular temperature over all wavelengths.