Heat Radiation

Absorption,Reflection and Transmission of heat

Introduction : When heat radiation is incident on a body, some part of heat radiation is reflected, transmitted and absorbed by the body. This amount of heat absorbed,t ransmitted and reflected depends on the coefficient of absorption, Transmission and reflection of heat from a body. Coefficient of absorption, transmission and reflection of heat radiation from the surface of the body depends upon the nature of material, area of material. 

Mathematically,

If Q is the total amount of heat incident on a surface of the body, Q_a is the heat absorbed by the body ,Q_t is the heat transmitted by the body and Q_r is the heat reflected by the body then

Q = Q_a + Q_t + Q_r -----(A)

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Coefficient of absorption (a): The coefficient of absorption of a body is defined as the ratio of the quantity of radiant energy absorbed by the body in given time to the quantity of radiant energy incident on the  body in the same time.

     a = Q_a  / Q  -----(1)

Coefficient of absorption  is also called  as absorptivity or absorptance. It is dimensionless quantity as it is the ratio of same quantity.

Coefficient of reflection (r): The coefficient of reflection of a body is defined as the ratio of the quantity of radiant energy reflected by the body in given time to the quantity of radiant energy incident on the body in the same time.

 a = Q_r  / Q  -----(2)

Coefficient of reflection is also called  as reflectivity or reflectance. It is dimensionless quantity as it is the ratio of same quantity.

Coefficient of transmission (t): The coefficient of transmission of a body is defined as the ratio of the quantity of radiant energy transmitted by the body in given time to the quantity of radiant energy incident on the body in the same time.

 a = Q_t  / Q  -----(3)

Coefficient of transmission is also called  as transmissivity or transmittance. It is dimensionless quantity as it is the ratio of same quantity.

Conservation of heat Energy: Total amount of heat radiation incident on a body is always equal to  sum of energy absorbed, reflected, transmitted.

From (A) ,

Q = Q_a + Q_t + Q_r

Dividing both side by Q,

Q/ Q =   Q_a /Q   +   Q_t /Q     +   Q_r  /Q 

1 = a+r+t ----(4)

from above equation it is clear that good absorber are poor reflecter and poor transmitter.

<<<Specific heat>>> │<<<Ferry's black body>>>

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