Vision Defects

An image is focused exactly on the retina for a person with normal vision. The common defects of vision are : short-sightedness, long-sightedness, astigmatism, colour blindness and presbyopia. 

Long-sightedness (Myopia)

Light ray which enter a normal eye will be focus on the retina.

When someone suffer from long-sightedness has the condition of unable to see the near objects clearly. The two causes: the eye lens is too thin or the eyeball is too short. Therefore, light from distant object is focused at the back of the retina (as shown in the diagram above). The correction method is to wear glasses with convex lenses. The lenses converge light rays before they enter the eye to form a sharp image on the retina.

  Short-sightedness (Myopia)

Can not view clearly for far object, the light focus in front of the retina

 

When someone suffer from short-sightedness has the condition of unable to see the distant objects clearly. The two causes: the eye lens is too tick and the eyeball is too long. Therefore, light from distant object is focused in front of the retina (as shown in the diagram above). The correction method is to wear glasses with concave lenses. The lenses diverge light rays before they enter the eye to form a sharp image on the retina.

 Colour Blindness

Due to shortage of cone cells on the retina. Most of the people who suffer from colour blindness can not differentiate between red and green colours. The situation can not be corrected because it is hereditary. People with colour blindness can not read "8" from the picture, because they can not differentiate green dots and red dots.

Astigmatism

 Astigmatism is caused by the irregular surface of the cornea or lens or both. The eye is unable to focus objects into a single, sharp image. Therefore, the images seen by astigmatic patient are distorted and not clear. To correct the situation, the patient have to wear glasses with cylindrical lenses, wear special contact lenses or undergoes surgery.

Limitations of sight

We can not see microscopic objects, distant objects, we can not see through opaque objects and when the light fall on the blind spot.

OPTICAL ILLUSION