2.1 Effects of Molecular Films
Consider a ray of light that is incident upon a surface that is contaminated with a thin film of thickness that is partially reflective and partially transmissive, Figure 2-1. Conservation of energy requires that the sum of the energies that are reflected back to space, , absorbed by the thin film, , and transmitted through the film, , be equal to the incident energy, . In terms of the normalized energies, this is where is the reflectance, is the absorptance, and is the transmittance. Because of the fundamental nature of materials, , , and will be functions of the angle of incidence, polarization, and wavelength of the incident energy. In general, absorptance may be inferred from experimentally determined values of reflectance and transmittance or from properties of bulk materials. Surfaces serving as mirrors or thermal radiators are usually made of materials that maximize reflectance and minimize transmittance. Baffles in optical and thermal systems require materials that absorb, or reflect, with a minimum of scattering. Other surfaces, such as solar array coverslides (broadband) or optical waveband filters (narrowband), are designed to maximize transmittance and minimize reflectance. As shown by Eq. 2-1, the absorptance of a clean surface satisfies the relation As will be seen shortly, in many problems of interest a surface is often designed so that either or is effectively zero. As approaches zero the surface becomes transparent, while as approaches zero the surface becomes opaque.