tio of the reflected radiant or luminous maintain sharp focus. This process is called accommodation. It is flux to the incident flux. 7/12 Optics and vision Reflection (optical) density (D) is the logarithm to the base ten of the reciprocal of the reflectance, as in Equation 7.7, where p (2) is the spectral reflectance. 10810( (7.7) 7.10.2 Absorption When a beam of light is propagated in a material medium its speed is less than its speed in a vacuum and its intensity gradually de- creases as it progresses through the medium. The speed of light in a material medium varies with the wavelength and this variation is known as dispersion. When a beam traverses a medium, some of the Figure 7.14 Linearly polarised light light is scattered and some is absorbed. If the absorption is true absorption the light energy is converted into heat. All media show some absorption. Some absorb all wavelengths more or less equally, sesses this property is said to be linearly polarised. Ordinary light, others show selective absorption in that they absorb some wave- such as that received from the sun or incandescent lamps, is unpo- lengths very much more strongly than others. The phenomena of larised and in this case the arrangement of vibrations is in all scattering, dispersion and absorption are intimately connected. possible directions perpendicular to the direction of travel, as in Figure 7.15. 7.10.2.1 Absorption coefficient Lambert's law of absorption states that equal parts in the same absorbing medium absorb equal fractions of the light that enters them. If in traversing a path of length dx the intensity is reduced from I to I - dI then Lambert's law states that dI/I is the same for all elementary paths of length dx. Thus Equation 7.8 may be obtained, where K is a constant known as the absorption coefficient. 1-=-Kdx (7.8) Therefore Equation 7.9 follows, where C is a constant. Log / = - K x + C (7.9) Figure 7.15 Unpolarised light f l = 1 at x = () then Equations 7.10 and 7.11 follow. There are numerous methods for producing linea