Properties of Real Surfaces — Lesson 3

This lesson covers the in-depth understanding of Kirchhoff's law in radiation heat transfer. It explains how the law relates the ability of a surface to absorb radiation with its ability to emit radiation. The lesson further elaborates on the conditions under which the hemispherical spectral emissivity equals the hemispherical spectral absorptivity. It also discusses the concept of diffuse isotropic surfaces and the impact of incoming radiation on these surfaces. The lesson concludes with a detailed explanation of the difference between absorptivity and emissivity, using a metal surface as an example.

Video Highlights

01:15 - Introduction to Kirchhoffprimes law and its relation to the ability of a surface to absorb and emit radiation.
06:20 - Explanation of the concept of a gray surface, whose radiative properties are independent of wave length.
15:49 - Discussion on the conditions under which hemispherical total emissivity can be equal to hemispherical total absorptivity.
21:58 - Explanation of the difference between absorptivity and emissivity, and how they can be measured indirectly by measuring reflectivity.
44:17 - Discussion on the conditions under which the directional total emissivity is linearly proportional to the surface temperature.

Key Takeaways

- Kirchhoff's law relates the ability of a surface to absorb radiation with its ability to emit radiation.
- The hemispherical spectral emissivity equals the hemispherical spectral absorptivity under certain conditions.
- Diffuse isotropic surfaces have properties that are independent of angle.
- The incoming radiation's angle independence is a rare occurrence in the real world.
- Absorptivity and emissivity are fundamentally different. While emissivity depends only on the surface temperature and nature, absorptivity depends on the environment and the incoming radiation.