• Fovea: This carries the greatest concentration of photoreceptors. During daytime it provides the sharpest point of vision. Most humans will automatically try to focus onto this point.
• Blind spot: This is the part of the eye that connects to the optic nerve, which means no photoreceptors are present and therefore no images can be formed here.
Visual problems Long-sightedness (hyperopia)
Long-sighted people can see distant objects clearly but objects close up are focused beyond the retina. This is mainly due to the cornea being too flat and the lens trying to adjust. As a person gets older, their ability to accommodate with their lens decreases and they tend to focus images beyond the retina (Fig. 15.18a). Long-sightedness is also called hyperopia.
Long-sightedness can be corrected by placing a convex (converging) lens in front of the eye, which increases the converging power of the overall eye system (Fig. 15.18b).
a Uncorrected light a
focus point beyond retina
Uncorrected light
focus point short of retina
Someone is long-sighted when they can see distant objects more clearly than close objects.
b Corrected with lens light
convex lens
FIG. 15.18 Convex lenses are used to correct long-sightedness
Short-sightedness (myopia)
Short-sighted people can see close objects clearly but have trouble focusing on distant objects. This is due to their eyes being too powerful in converging light (Fig. 15.19a). Short-sightedness is also called myopia. By placing a concave (diverging) lens in front of the eye, its power of convergence is reduced (Fig. 15.19b).
focal point on retina
b Corrected with lens light
concave lens
FIG. 15.19 Concave lenses are used to correct short-sightedness
