Why is radiation usually treated as a surface phenomenon?

Radiation is an energy transfer mechanism through electromagnetic waves, which include visible light, infrared radiation, ultraviolet radiation, microwaves, and radio waves. Unlike conduction and convection, which require a material medium to occur, radiation can travel through empty space or a vacuum. This is possible because electromagnetic waves are driven by oscillating electric and magnetic fields.

When radiation reaches a surface, several interactions can occur. These include absorption (where the surface absorbs the radiation energy and converts it into heat), reflection (in which the radiation is reflected back into the surrounding, without transferring its energy to a surface), and transmission (where radiation passes through a material, like a glass window, without being absorbed or reflected). The extent of each interaction depends on the properties of both the radiation and the surface.

Surfaces emit radiation as well. The energy emitted is related to the temperature of the surface, and it is governed by the Stefan-Boltzmann law, which states that the energy emitted per unit area by a black body is proportional to the fourth power of its absolute temperature (\(E = σT^4\), where \(σ\) is the Stefan-Boltzmann constant and \(T\) is the temperature). Real surfaces will have an emissivity (\(ε\)) factor that indicates how much they deviate from a perfect black body, with the actual emission being \(E = εσT^4\). The emissivity depends on surface material and finish.

Due to the aforementioned properties and interactions, radiation is mainly considered as a surface phenomenon for the following reasons: 1. Radiation exchange occurs at the surfaces of objects. Surfaces will emit, absorb, reflect, and transmit radiation depending on their characteristics and the incident radiation. This leads to the need to analyze radiation from a surface perspective. 2. The energy emitted by the surfaces is a significant factor in determining the net heat transfer between these surfaces. In particular, the radiative properties, temperature, and orientation of the surfaces all play a crucial role in predicting this heat transfer. 3. Surface properties and interactions often dominate the overall radiative heat exchange in a system, so considering radiation as a surface phenomenon helps simplify the problem and allows engineers and scientists to focus on relevant factors that influence radiation heat transfer. Thus, the surface interactions and emission processes play a vital role in understanding the behavior of radiation, and this is why radiation is typically regarded as a surface phenomenon.