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(i) Perfectly black body: A system that absorbs the entire radiation incident upon it and has an emissivity same to 1 is known as a perfectly black body. A black system is also an ideal radiator. It shows that if a black body and an identical another system are kept at the similar temperature, then the black body will radiate maximum power as it is obvious from equation P = eAσT4 also. Because e = 1 for a perfectly black body while for any other system e < 1.
Radiation entering the cavity has little advantage of leaving before it is completely absorbed. (e ≈ 1)
Materials like lamp black or black velvet come close to being ideal black systems, but the best practical realization of an ideal black object is a small hole leading into a cavity, as this absorbs 98% of the radiation incident on them.
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Cavity approximating an ideal black body.
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(ii) Absorptive power 'a': "It is described as the ratio of the radiant energy absorbed by it in a given time to the total radiant energy incident on it in the same interval of time."
a = energy absorbed/energy incident
As a perfectly black body takes all radiations incident on it, the absorptive energy of a perfectly black object is maximum and unity.
(iii) Spectral absorptive power 'al': The absorptive power 'a' denotes to radiations of all wavelengths while the spectral absorptive power is the ratio of radiant energy given by a surface to the radiant energy incident on it for a particular wavelengthl. It can have different values for different types of wavelengths for a given surface. Let us consider an example, suppose a = 0.6, aλ = 0.4 for 1000 Å and aλ = 0.7 for 2000 oA for a given surface. Then it defines that this surface will absorbs only 60% of the net radiant energy incident on it. Similarly it absorbs 40% of the energy incident on it corresponding to 1000 oA and 70% corresponding to 2000 oA. The spectral absorptive energy al is related to absorptive power a through the relation
(iv) Emissive power 'e':.
"For a given surface it is described as the radiant power emitted per second per unit area of the field." It has the units of W/m2 or J/s-m2. For a black box e=σT4.
(v) Spectral emissive power 'el': "It is emissive energy for a particular wavelengthl." Thus,
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