1. Modes of Heat Transfer

Thermal radiation incident upon a surface may be either absorbed, reflected, or transmitted. 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.

• The relation \(\alpha = \epsilon\) is known as Kirchhoff’s law of radiation.

• Surfaces with emissivities nearly unity are good absorbers and hence poor reflectors of incident radiation. Most highly polished, unoxidized metal surfaces are good reflectors of thermal radiations with emissivities less than 0.1. A roughened or an oxidized surface has correspondingly higher emissivities.