Investigation and previous work on optical and thermal measurements in transparent materials - Dissertation Example

The optical and thermal measurements of Aluminum are also discussed because a part of this dissertation involves the analysis of Aluminum metal as a reflector for heat dissipation. When light rays hit the boundary of a refractive material, they are split into subrays according to the solid angle caused by the dispersion and the distance traveled by the split rays until they hit the next surface of the material (Wilkie, Tobler, & Purgathofer, 2000). Ray tracing is based on this principle and is used to study optical materials.

When radiation falls on the surface of a material, some part of it is reflected, some is absorbed, and some of it is transmitted. These three are the most important optical features of a material and are characterized as the reflectivity (?) of the surface of the material, absorbance (?) of the material, and transmissivity (?) of the material (Bartl & Baranek, 2004). These three optical features of a substance are related to each other as per the equation: ? +? +? = 1. The refractive index of a material is given by the ratio of the speed of light in vacuum to the speed of an electromagnetic wave in the material.

The refractive index is an important optical property. . transitions, multiphoton processes, scattering and defect and impurity absorption, density variations, etc are important loss mechanisms that greatly influence the optical properties of the substance (Tropf, Thomas, & Harris, 1995). Lattice vibrations or atomic motion in the material are responsible for its optical properties, dielectric properties, heat capacity, thermal conductivity and other important thermo-optic properties (Tropf, Thomas, & Harris).

The refractive index, an important optical property, is influenced by a number of factors including temperature, stress, and applied field. The study of temperature effects on the refractive index of a material, called thermo-optic properties of the material, is especially important in case of solar concentrators because they are continuously exposed to heat and high temperatures due to exposure to sunlight. Thermal measurements such as measurements of thermal expansion and thermal conductivity of optical materials are thus important for predicting its behaviour.

The coefficient of linear thermal expansion, given by ?, is the fractional change in the length with respect to change in temperature (Tropf, Thomas, & Harris, 1995). It is thus defined by the equation: ? (T) = 1/L dL/dT The thermal conductivity of a material, given by ?, is the “rate of heat flow” through the substance within a specific “thermal gradient” (Tropf, Thomas, & Harris). Figure 3.1 shows the thermal conductivity of several common optical materials. Fig. 3.1: Thermal conductivity of optical materials (Tropf, Thomas, & Harris, 1995, p. 33.37).

Studying the optical and thermal behaviour of optical materials is an important prerequisite in order to utilize them in solar applications. Several methods for carrying out optical and thermal