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Interaction of electromagnetic radiation with matter

Introduction

EM radiation interacts with matter in several ways. EM radiation is emitted by matter. Further, the EM radiation that reaches an object interacts with it. As a result of this interaction, EM radiation is absorbed, transmitted or reflected by the object.

The energy conservation law, applied to interaction of EM radiation with the object, states that all incident EM radiation (I) is absorbed (A), reflected (R), or transmitted (T):

A(%5Clambda)%2BR(%5Clambda)%2BT(%5Clambda)%3DI(%5Clambda)
Equation 1


Dividing both sides of the Equation 1 by I we get

%5Cfrac%7BA(%5Clambda)%7D%7BI(%5Clambda)%7D%2B%5Cfrac%7BR(%5Clambda)%7D%7BI(%5Clambda)%7D%2B%5Cfrac%7BT(%5Clambda)%7D%7BI(%5Clambda)%7D%3D%5Calpha(%5Clambda)%2B%5Crho(%5Clambda)%2B%5Ctau(%5Clambda)%3D1
Equation 2
 
where α(λ) is absorptance, ρ(λ) is reflectance and τ (λ) is transmittance of the object, all depend on wavelength λ and range from 0 to 1.

It is important to note that Equations 1 and 2 apply for each wavelength.

For opaque objects τ (λ) = 0 and Equation 2 reduces to

%5Calpha(%5Clambda)%2B%5Crho(%5Clambda)%3D1

Equation 3

Absorption of EM radiation leads to an increase in the object’s temperature, while emission of EM radiation leads to a decrease in the object’s temperature. The amount of emitted EM radiation is determined by the object’s temperature (see Planck’s law) and emissivity (λ). In equilibrium the total amounts of absorbed and emitted radiation at all wavelength are equal and the object’s temperature is constant.

Kirchhoff’s law of thermal radiation states that in equilibrium absorptance and emissivity at each wavelength are equal

%5Calpha(%5Clambda)%3D%5Cepsilon(%5Clambda)
 

The reflectance, transmittance and absorptance vary with wavelength and type of target material. Here and further in the book we define a target as an object on the Earth surface that is being detected or sensed. Also the surface of target influences interaction of EM radiation and the target. Two types of reflection that represent the two extremes of the way in which radiation is reflected by a target are “specular reflection” and “diffuse reflection” (Figure 1). In the real world, usually a combination of both types is found.

Figure 1. Schematic diagrams showing (a) specular and (b) diffuse reflection.
  • Specular reflection, or mirror-like reflection, typically occurs when a surface is smooth and (almost) all of the radiation is directed away from the surface in a single direction. Specular reflection can occur, for example, for a water surface or a glasshouse roof. It results in a very bright spot (also called “hot spot”) in the sensed image.
  • Diffuse reflection occurs in situations where the surface is rough and the radiation is reflected almost uniformly in all directions.

Whether a particular target reflects specularly, diffusely, or both, depends on the surface roughness relative to the wavelength of the incident radiation.

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