1. An eye is the most important organ of the human body. It is associated with vision and acts as a vital optical instrument.
2. An eye is spherical in shape of about 2.3 cm diameter and is covered externally by a hard thick and opaque layer called Sclerotica. The outermost layer of the eye is sclerotic that prtects and holds the eye.
3. Cornea is transparent part bulging out of eye and allows the light to enter in the eye. Cornea is also known as the window or aperture of the eye.
4. Iris is a circular diaphragm having a hole in its center .this hole is called pupil
5. Iris has muscles and coloured pigments that provide different colour y eyes
6. Iris controls the amount of light entering the eye through the pupil.
7. The pupil becomes small when bright light falls on the eye. However, it becomes wide when there is dim light.
8. The eye lens is a crystalline convex lens made up of transparent and flexible tissues behind the pupil and held by the muscles called ciliary muscles. It focuses inverted the images of objects on the retina of the eye.
9. Ciliary muscles control the focal length of the eye lens by contracting and relaxing. This ability
10. When ciliary muscles contract lens become thin thus the focal length of the lens increases.
11. When ciliary muscles expand lens become thick, and decrease the focal length of the lens. This helps us to see nearby objects clearly.
12. The ability of the eye lens to adjust its focal length is called accommodation.
13. The focal length of the eye lens cannot be decreased below a certain minimum limit this is why a normal eye not able to see clearly the objects placed closer than 25 cm.
14. The image distance in the eye remains same when we increase the distance of an object from the eye as distance between eye lens and retina is constant.
15. The least distance of distinct vision for a young adult with normal vision is about 25 m. it is called near point
16. The furthest distance of distinct vision for a young adult with normal vision is at infinity. it is called far point.
17. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to accommodation.
18. A normal eye can see objects clearly that are between 25 cm and infinity.
19. In old age eye lens of people becomes milky and cloudy. This condition is called cataract. This causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.
20. One eyes of human being has a horizontal field of view of about 150° So for a horizontal field of view of about 150° 180° We have two eyes.
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