1. Define emissivity of a surface. Explain spectral, direction al, hemispherical and total emissivity. 2. What is a gray body? How does vary for a gray body and for a real surface? 3. Explain...

1. Define emissivity of a surface. Explain spectral, direction al, hemispherical and total emissivity.

2. What is a gray body? How does
vary for a gray body and for a real surface?

3. Explain Kirchhoff’s law. What do you mean by the statement: A perfect absorber of radiant energy is also a perfect emitter?