Linear Optical Properties

Abstract

This chapter provides an extended overview of the optical properties of glasses. In Sect. 5.1 the underlying physical background of light–matter interaction is presented, where the phenomena of refraction, reflection, absorption, emission and scattering are introduced.

Most oxide glasses are transparent in the visible spectral range. This obvious fact, confirmed by every look through a window, is based on two highly nontrivial principles: (i) the existence of an electronic bandgap and (ii) the (nearly) complete absence of light scattering. Although transparent solid materials like single crystals and glasses have been known for thousands of years, understanding the existence of an electronic bandgap, an energy range where practically no absorption of electromagnetic radiation occurs, requires quantum mechanics, which just became 100 years old. If such a forbidden zone is larger than the photon energy of blue photons, the photons with the largest energy quantum in the visible spectral range, the material is visibly transparent.

$$E_{\text{band\_gap}}> h\nu_{\text{blue}}\;,$$

with $h\nu_{\text{blue}}=hc/\lambda_{\text{blue}}={\mathrm{3.18}}\,{\mathrm{eV}}$, where $h$ is Planck's constant, $c$ is the speed of light in a vacuum (or air), $\lambda$ is the wavelenght and $\nu$ is the frequency. The energy is given here in units of eV (${\mathrm{1}}\,{\mathrm{eV}}={\mathrm{1.602\times 10^{-19}}}\,{\mathrm{J}}$).

The (nearly) complete absence of light scattering in glasses has its origin in the fact that as opposed, e. g., to most ceramic materials, glasses are isotropic and extremely homogeneous on all length scales relevant for the interaction with visible light. Also, while glasses have well-defined structure on the atomic scale, a few ångstroms ($\mathrm{\AA}$), they are completely disordered and therefore homogeneous and isotropic on the larger length scales relevant for the interaction with visible light.