- As we know that, if radiant energy is incident on the surface of any body then the incident radiant energy may be partly absorbed, partly emitted, partly transmitted and partly get reflected.
- This ability of the body to absorb, reflect, transmit and emit the incident radiant energy is measured in terms of coefficient of absorption, coefficient of reflection, coefficient of transmission and coefficient of emission respectively.
- Thus, we can write an equation as
- Q = Qa + Qr + Qt
- Where, Qais the absorbed radiant energy
- Q is the reflected radiant energy
- Qt is the transmitted radiant energy
Statement:
According to Kirchhoff’s law of radiation, the coefficient of absorption of the body is always equal to the coefficient of emission at a given temperature of the body.
Thus, we can write
Hence, a = e
But we know that,
Coefficient of emission e = E/Eb
And hence,
Coefficient of absorption a = E/Eb
Thus, Eb = E/a
Hence, we can again define the Kirchhoff’s law of radiation as, the ratio of the emissive power of the body E to its coefficient of absorption a is constant and this constant is nothing but the emissive power Eb of the body at that temperature.
Explanation:
Coefficient of absorption:
- Coefficient of the body is defined as it is the ratio of the amount radiant energy absorbed Q to the total radiant energy incident on it.
- And hence it can be given as, a = Qa/Q
- This coefficient of absorption a is also called as absorptivity or absorptance.
- More is the black surface more will be the absorption.
- Hence, the coefficient of absorption for perfectly black body is 1.
Emissive power:
- The emissive power of the body is defined as it is the radiant energy emitted by the body per unit time per unit surface area of the body at a given temperature.
- Hence, emissive power can be given as
- E = Q/At
- The SI unit of emissive power is J/m2s or W/m2.
- And the dimensions of emissive power will be [M1 L0 T-3].
- The emissive power of the body depends on mainly following factors:
- temperature of the given body
- nature of the given body
- nature of the surroundings
- surface area of the body
- The emissive power of the perfectly blackbody is greater than any other bodies always.
For example:
Lampblack is having emissive power as 98%.
Coefficient of emission:
- The coefficient of emission of the body is defined as it is the ratio of the emissive power of the body at a given temperature to the emissive power of the perfectly blackbody at the same temperature.
- Thus, it can be written as
- Coefficient of emission e = E/Eb
Where E -is the emissive power of the body at given temperature
Eb -is the emissive power of the perfectly blackbody at given temperature
- For perfectly blackbody coefficient of emission e = 1
- For perfect reflector the coefficient of emission e = 0
- For ordinary bodies the coefficient of emission e < 1