41 - Radiation

When thermal radiation is incident on a surface, a part of the radiation may be reflected by the surface, a part may be absorbed by the surface and a part may be transmitted through the surface as shown in figure.

If \(\rho, \alpha,\) and \(\tau\) are the fractions of the incident radiation which are reflected, absorbed and transmitted, respectively, then \[\rho + \alpha + \tau = 1\] where \(\rho\) is reflectivity, \(\alpha\) is absorptivity, and \(\tau\) is transmissivity.