The mammalian eye could be regarded as a miracle of evolution. In order to produce an image the eye has to act as an Optical refracting System (to focus a sharp image) and a Photo Detector (to analyse the image and send information to the brain). At the front of the eye is the optical system, made up of the transparent curved Cornea and the adjustable biconvex Lens. The photo detector is the layer of light-sensitive cells at the back of the eye, called the Retina. The amount of light entering the eye is controlled by the Iris. Sharp focusing is achieved by altering the shape of the lens. The shape of the lens is controlled by the ring of ciliary muscle which runs round the outside of the lens.
Defects in the eye:-
1. Long Sight (Hypermetropia)
In this condition we can see distant objects more clearly that ones that are nearby. This is usually because the cornea is not curved enough, which reduces the angle of refraction, and the lens cannot accommodate sufficiently to bring the diverging rays from near objects into sharp focus on the retina.
2. Short Sight (Myopia)
In this condition near objects are clearly seen, but distant objects cannot be brought into the focus. It happens when the cornea is too curved; the light from distant objects is refracted too much and comes to a focus in front of retina.
If the cornea is curved in an uneven manner, it will refract light that enters the eye in different planes to different extents. The eye will focus light in some planes better than others (i.e. verticals might be in focus, but horizontals not)
This is the natural reduction in the lens’s accommodation power with the age. (Accommodation is the process of changing the shape of the lens in order to focus the sharp image on the retina.)
* Long Sight (Hypermetropia)
The situation can be corrected by placing a converging lens (convex lens) in front of the eye (by means of spectacles of contact lenses). In extreme cases laser surgery can be used to correct the shape of the cornea.
* Short Sight (Myopia)
This situation can be corrected by placing a diverging lens (concave lens) in front of the eye. In extreme cases laser surgery can be used to correct the shape of the cornea.
This situation can be corrected by using a cylindrical lens (i.e. one that only curves in one plane)
This can be corrected by using bifocal lenses, which have different curvatures in different regions. While looking at close objects we tend to look down, so the lower region of the lens is manufactured to correct for defects in short sight. When we look at distant objects we tend to look up, so the upper part of the lens caters for long-sight problems.