Human Eye:
Works like a photographic camera. Image is formed at the retina of the eye.
Range of Vision: 25cm to infinity.Defect of Vision
1. Myopia: Short-sightedness (Can see nearest objects), person can not see the distant (दूर) objects clearly.
- Image is formed before retina.
- Concave lens is used to correct myopia.
2. Hypermyopia: Long-sightedness (Can see far objects), person can not see near objects.
- Image is formed behind retina.
- Convex lens is used to correct hypermetropia.
3. Presbyopia: In old age, the power of accommodation of eye lens decrease hence one can not see both near and distant objects.
- Bifocal lens is used to correct presbyopia.
4. Astigmatism: Image formed is not clear, because "Cornea" becomes irregular.
Corrected by: Cylindrical lens.
Eye related Problem
Cataract: मोतियाबिंद
An opaque, white membrane is developed on cornea, this cloud result in blurred images. The person loses vision partially or completely.
- Can be covered by getting membrane removed by surgery.
Glaucoma
It is caused by intraocular pressure usually resulting from malfunction in the eye's drainage system. Causes damage to optic nerve and can lead to permanent vision loss.
- Can be treated by surgery.