JEE Main & Advanced Physics Transmission of Heat Interaction of Radiation with Matter

When thermal radiations (Q) fall on a body, they are partly reflected, partly absorbed and partly transmitted.

(1) \[Q={{Q}_{a}}+{{Q}_{r}}+{{Q}_{t}}\]

(2) \[\frac{{{Q}_{a}}}{Q}+\frac{{{Q}_{r}}}{Q}+\frac{{{Q}_{t}}}{Q}=a+r+t=1\]

(3) \[a=\frac{{{Q}_{a}}}{Q}\]= Absorptance or absorbing power

\[r=\frac{{{Q}_{r}}}{Q}\] = Reflectance or reflecting power

\[t=\frac{{{Q}_{t}}}{Q}\]= Transmittance or transmitting power

(4) r, a and t all are the pure ratios so they have no unit and dimension.

(5) Different bodies

(i) If \[a=t=0\] and \[r=1\to \] body is perfect reflector

(ii) If \[r=t=0\] and \[a=1\to \] body is perfectly black body

(iii) If, \[a=r=0\] and \[t=1\to \] body is perfect transmitter

(iv) If \[t=0\Rightarrow r+a=1\,\,\text{or}\,\,a=1-r\]  i.e. good reflectors are bad absorbers.