The optical properties of semiconductors are directly related to the dielectric constant of the material. This dielectric constant gives the ratio between the electric permeability of a material in relation to the permeability of a vacuum. The imaginary refractive index of a material is given by the square root of the dielectric constant. The reflectance  of a material can be calculated using the Fresnel equations. A present electric field alters the dielectric constant and therefore alters the optical properties of the material, like the reflectance.