A material may transmit, reflect, emit or absorb radiation, and generally does more than one of these at a time. These properties have spectral and geometric characteristics that are described by appropriate adjectives. The properties come in pairs. Transmission and reflection differ only in directionality. In a very real sense they are only 180 degrees apart. By Kirchhoff's law it can be shown that, under identical conditions, the emission of a body is equal to its absorption.
4.1 Some Definitions
This is the section on ivities, ances, and ions. It is generally accepted that the ending ion signifies a process. Emission is the process of radiating. Transmission is the process of transmitting. The same with absorption and reflection. An ance ending is said to indicate the property of a particular sample. Thus, one reports the reflectance of sample 2306 as measured on April Fool's Day. The ivity ending is meant to signify the property of the âgeneric brand,â an idealized sample that represents all such samples of that material. There has been controversy about whether there is a meaningful difference between a reflectance and a reflectivity. For reflectivity, the sample must be pure, clean, and smooth enough that the reflection is not affected by any lack thereof. But a body always has some roughness, and the effects of this roughness always can be measured. In this text the ance and ivity forms of these words will be used interchangeably, but the reader should understand that some workers make a distinction (or distinctance or even distinctivity!).
4.2 The Conservation of Power
Figure 4-1 shows light incident on a plane parallel plate. Some of it is reflected, some is absorbed, and some is transmitted. The sum of these powers must add up to the incident power if power conservation is to be maintained.
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