shifted towards the blue end of the spectrum infinity. from 555nm to 507nm. When the eye is adapted to very dim light, rod vision is dominant. 7.9.2 Short sightedness The process of adaptation where the eye brain combination auto- matically adjusts the sensitivity of the system to light allows com- Myopia is the condition of the eye where parallel rays are brought fortable viewing of scenes differing in luminance by about a factor to focus in front of the retina. The eye lens is unable to relax of 1011. If one compares the minimum intensities which effectively sufficiently to focus parallel light. On the other hand when the switch on rod vision and cone vision then it is found that rod vision ciliary muscles are tensed the eye can usually see objects much is 100 to 1000 times more sensitive than cone vision. This is closer to the eye than normal. Hence the name of the defect short illustrated in Figure 7.13 where the relative spectral sensitivity of sightedness. For correction a concave spectacle lens enables the eye rod and cone vision is shown. to see distant objects because it makes the light diverge as though from a less distant object. 7.9.3 Far sightedness ROD VISION Hypermetropia is the condition where the eye lens is unable to focus on objects close at hand. The condition is called far sight in that distant objects are seen more clearly than closer ones. The ciliary muscles cannot contract sufficiently to bring the image of near objects on to the retina. For correction a convex spectacle lens converges light from a near object and makes it appear to come from CONE VISION a more distant position which the eye can cope with. With advancing years, the range of accommodation reduces until myopia or hypermetropia or even both are present. This may necessitate two pairs of spectacles or a single pair of bifocal lenses. WAVELENGTH (nm) 7.9.4 Astigmatism Many people suffer from some degree of astigmatism in the middle Figure 7.13 Relative spectral sensitivity of cone and rod of their visual field, mainly due to p