Transparency and Translucency - Introduction

Introduction

With regard to the absorption of light, primary material considerations include:

• At the electronic level, absorption in the ultraviolet and visible (UV-Vis) portions of the spectrum depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific frequency, and does not violate selection rules. For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible light, or if they do, they violate selection rules, meaning there is no appreciable absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.
• At the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds, and on selection rules. Nitrogen and oxygen are not greenhouse gases because the absorption is forbidden by the lack of a molecular dipole moment.

With regard to the scattering of light, the most critical factor is the length scale of any or all of these structural features relative to the wavelength of the light being scattered. Primary material considerations include:

• Crystalline structure: whether or not the atoms or molecules exhibit the 'long-range order' evidenced in crystalline solids.
• Glassy structure: scattering centers include fluctuations in density or composition.
• Microstructure: scattering centers include internal surfaces such as grain boundaries, crystallographic defects and microscopic pores.
• Organic materials: scattering centers include fiber and cell structures and boundaries.