Optical Properties
 Absorption, Refraction, Reflection

Light is energy in the form of radiation. It is regarded as being composed of either photons in the energy range (2.5 to 5.6 x 10^-19) or electromagnetic waves in the wavelength range of 800 nm down to 380 nm (see Figure 9-12). Light, being a form of electromagnetic radiation, interacts with the electronic structure of atoms of a material. The initial interaction is one of absorption; that is, the electrons of atoms on the surface of a material will absorb the energy of the colliding photons of light and move to the higher-energy states. The degree of absorption depends, among other things, on the number of free electrons capable of receiving this photon energy. The electrons can then do several things. They can have sufficient energy to jump into higher-energy states, which permits them to be accelerated within an electric field and thereby conduct electricity (photoconductive effect). They can collide with atoms and release their excess energy in some form of electromagnetic radiation, or they can convert their excess energies to atoms in the form of thermal energy.

As light enters a nonmetallic material, it is absorbed (Figure 9-13) by the action of several mechanisms, such as electronic polarization (Figure 9-5) and electron transitions. The electron transitions depend on the electron energy band structure of the material. The interaction process characteristic of photons depends on their energy (E^_r). Low-energy photons interact principally by ionization or excitation of the outer orbitals in a solid's atoms. Light is composed of low-energy photons (E^_r is less than 10 eV) represented by infrared (IR), visible light, and ultraviolet (UV) in the electromagnetic spectrum. (See Figure 9-12.) UV light is being tested for curing solvent-free urethane paint on autobodies as a technique to eliminate the release of VOCs into the atmosphere from the spraying as well as the costly baking process newly painted autobodies must undergo. High-energy protons (E^_r greater than 10⁴ eV) are produced by x-rays and gamma rays (Figure 9-14). The minimum energy (E^_r) required to excite and/or ionize the component atoms of a solid is called the absorption edge or threshold.

When crystalline materials are exposed to radiation, the change in their properties is due to a displacement of atoms from their normal locations in the crystal structure, i.e., the creation of crystal defects. The total energy of the incident radiation is measured in terms of dose, the amount of radiation affecting the material, or the dose rate, which is the rate of energy deposition. Irradiation conditions are often given in terms of the total number of particles incident on the sample, i.e., the fluence (fluence equals the summation of flux, or the particles per unit time deposited, over irradiation time) [see Figure 9-13(c)]. A greater understanding of this topic requires a study of the mechanisms by which energy is transferred from the radiation (in this case, light) to the atoms of a solid. In the case of metals, only the energy transferred to the nuclei by the beam of energetic, charged particles such as electrons, photons, and fission fragments is effective in creating lattice defects. The crystalline structure can also be converted to an amorphous one by sufficient exposure to radiation. In metals, the band gap is zero and photons of all energies can be absorbed to increase the kinetic energy of the conducting electrons and holes. In polymers (covalently bonded materials), photons of suitable energy are capable of exciting vibrations directly in both molecules and crystalline structure.The chemical bonding can be altered by electronic excitation, which can also create cross-links and break primary bonds.

When light is transmitted into the interior of a transparent solid, it experiences a decrease in its velocity. The result is the light bends as shown in Figure 9-13a. This phenomenon is known as refraction. The index of refraction (n) of a solid material depends on the wavelength of the incident light. For air, the index is nearly equal to 1, whereas water has a higher index of refraction, 1.33. Other indices of refraction for some materials are as follows: silica glass, 1.46; corundum, 1.76; polyethylene, 1.51; and polypropylene, 1.49. Lucite (Plexiglas) has an index of 1.51 and diamond's is 2.42. When light passes from one medium to another having a different index of refraction, some of the light is scattered at the interface between the two media. Both of these media could be transparent materials. The term reflexivity (R) is the ratio of the intensity (I) of the reflected light to the intensity of the incident light (I_R / I_o) that is reflected at the interface (see Figure 9-15).