The human eye allows us to see by allowing light to enter through the cornea and lens, which focuses the light onto the retina. The lens adjusts its curvature, a process called accommodation, to focus on objects at different distances, with the help of ciliary muscles. The retina contains light-sensitive cells that detect the image and send signals to the brain via the optic nerve. Some common vision problems include myopia, where the eye focuses images in front of the retina, hypermetropia where images are focused behind the retina, and presbyopia which occurs due to weakening muscles with age.

1. THE HUMAN EYE DIVYANK GUPTA Xth – E 16 .

2. CROSS SECTION THROUGH HUMAN EYE

6. POWER OF ACCOMMODATION The ability of the eye lens to adjust its focal length is called accommodation. The eye lens is composed of a fibrous, jelly-like material and its curvature can be modified by the ciliary muscles. Hence, the focal length can be changed as per the requirement. When the muscles are relaxed, the lens becomes thin. The radius of curvature and hence the focal length increases. This enables us to see the distant objects clearly. When we look at the objects closer to they, ciliary muscles contract decreasing the radius of curvature and hence the focal length. This enables us to see the nearby objects clearly.

7. LEAST DISTANCE OF DISTINCT VISION (LDDV): The minimum distance, at which objects can be seen most distinctly without strain, is called Least Distance of Distinct Vision(LDDV). For a normal eye, LDDV is 25 cm.

8. BENEFITS OF TWO EYES FOR VISION There are several advantages of our having two eyes instead of one. It gives a wider field of view . a human being has a horizontal field of view of about 150 degree with one eye and of about 180 degree with two eyes . The ability to detect faint objects is , of course , enhanced with two detectors instead of one . Our two eyes our positioned on the front of our heads , and it thus reduces our field of view in favour of what is called stereopsis . Our brain combines the two images into one.

11. Myopic Eye Near Point MYOPIC EYE CORRECTED WITH CONCAVE LENS Normal Eye LDDV = 25 cm O LDDV = 25 cm I O LDDV = 25 cm O I I O LDDV = 25 cm I O LDDV = 25 cm I I

12. HYPERMETROPIA OR LONG-SIGHTEDNESS OR FAR-SIGHTEDNESS A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. Such a person may has to keep a reading material much beyond 25 cm from the eye for comfortable reading. In hypermetropic eye, the image of a nearby object is formed behind the retina and not on the retinal itself. This defect may arise due to : (i) long focal length of the eye lens or (ii) Very small size of the eyeball. Hypermetropia can be corrected by using a convex lens of suitable power (focal length).

13. Hypermetropic Eye Near Point HYPERMETROPIC EYE CORRECTED WITH CONVEX LENS Normal Eye LDDV = 25 cm O LDDV = 25 cm I O O LDDV = 25 cm I I LDDV = 25 cm I O LDDV = 25 cm O I I