Human Eye
Light enters the eye through the cornea. Because of its curvature and high refractive index (1.38), most of the refraction of light takes place at the front surface of the cornea. Light then passes through a liquid, aqueous humour to reach the lens. A coloured iris in front of the lens controls the size of the pupil which in turn controls the light intensity. The lens is connected to muscles which can change its shape, allowing it to focus the image. After leaving the lens the light passes through the vitreous humour, which helps maintain the shape of the eye, and reaches the retina where the image is formed and detected by light sensitive cells. This information is then carried by the optic nerve to the brain.
Defects in vision such as nearsightedness, farsightedness or astigmatism are related to the shape of the eye or the lens. In nearsightedness (myopia), the image forms in front of the retina because the eyeball is too long or the lens is too convex. This can be corrected by using diverging lenses. In farsightedness the focus point of the light is behind the retina. This is caused by the lens being too flat or the eyeball too short. Farsightedness can be corrected by using a converging lens. Astigmatism occurs when the cornea or the lens of the eye is not perfectly spherical. As a result, the eye has different focal points in different planes. This can be corrected by having glasses with the proper curvatures.
Test for Astigmatism
Those who have farsightedness (hyperopia) are able to see distant objects but have difficulty focussing on nearby objects. In this case, the image is formed behind the retina. This can be corrected by using a converging lens that causes the light rays to bend more and focus the light rays on the retina as shown in figure 8.
The eye is connected to the brain by the optic nerve which transmits information about the image formed on the retina to the brain. Even though the image formed on the retina is upside down, we see things upright because our brain interprets the information and turns the image upright.
The part of the retina where the optic nerve enters the eye is not sensitive to light. That area of the retina is known as the blind spot. To find your blind spot, do the blind spot activity.
Correcting Vision
The lens in your eye can change shape by the use of small muscles in the eye. By changing shape, the lens is able to focus the image on the retina at the back of the eye. For objects that are far away the lens becomes more flat and for objects that are close the lens becomes more rounded and thicker. See figure 6. (The light is also refracted as it passes through the cornea but for simplicity that affect has been left out of the diagram.)
People with nearsightedness (myopia) can see objects that are near but have trouble focussing on distant objects due to the shape of the eye or the lens. The image is focussed in front of the retina rather than on the retina and as a result the image on the retina is blurred. Nearsightedness is corrected by using eyeglasses (or contact lenses) that diverge the light rays so that the image will now reach the retina (as shown in figure 7.