Optics is the study of light motions including reflection, refraction, diffraction, and interference. Different frequencies undergo different angles of refraction, a phenomenon known as dispersion. Single calcite crystals display an optical property called birefringence (double refraction). True tombolos are formed by wave refraction and diffraction. The radiance divided by the index of refraction squared is invariant in geometric optics. The book contains detailed descriptions of phenomena such as coloured shadows, refraction, and chromatic aberration. Examples of physical properties include density, index of refraction, magnetization and chemical composition. Atmospheric refraction near the ground produces mirages. These professionals specialized in optics and refraction. In it, Ptolemy writes about properties of light, including reflection, refraction, and colour. This effect resembles the refraction of light waves. In a conducting medium, permittivity and index of refraction are complex-valued. The formula also neglects the effect of atmospheric refraction. Consequently, so are the angle of refraction and the wave-vector. Green flashes are enhanced by mirages, which increase refraction.