describes the electrical and optical properties versus frequency, wavelength, or energy. It describes the polarization (electric polarizability) and absorption properties of the material. The dielectric function is commonly denoted by the Greek letter epsilon, ε, and consists of two parts called epsilon 1 and epsilon 2 written as ε1 and ε2, but often written simply as e1 and e2. Since the quantities e1 and e2 vary with wavelength, frequency or photon energy it is common to write them as functional expressions such as e1(λ) & e2(λ), or e1(E) and e2(E), etc.