Defect of vision

slide on Defect of eye or vision

1. DEFECTS OF VISION

2. INTRODUCTION HUMAN EYE Human eye is the optical instrument used which enables us to see. It acts like a camera, enable us to capture the colourful picture of the surroundings. An inverted, real image on light sensitive is formed on the Retina

3. Cornea Eyeball Iris Pupil Crystalline eye lens Ciliary muscles Retina

4. CORNEA It is a thin membrane through which light enters. It forms the transparent bulge on the front of eyeball. Most of the refraction occurs at the outer surface of the cornea.

5. Eyeball It is a convex lens, approximately spherical in shape, with a diameter of about 2.3cm. It can alter its curvature with help of ciliary muscles.

6. Iris It is a dark muscular diaphragm that controls the size of pupil . It is behind the cornea. I t helps in accommodation of light by changing the size of the pupil. IRIS

7. Pupil It regulates and control the amount of light entering the eye. It is the black opening between aqueous humor & lens. Black in colour . Light entering cannot exit. PUPIL

8. Crystalline eye lens Provides the focused real & inverted image of the object on the retina. It is composed of a fibrous, jelly like material. This is convex lens that converges light at retina.

9. Ciliary muscles It helps to change the curvature of eye lens and hence changes its focal length so that we can see the object clearly placed at different positions .

10. Retina Thin membrane with large no. of light sensitive cells. There are two types of photoreceptors in the human retina, rods and cones. Rods are responsible for vision at low light levels . They do not mediate colour vision . Cones are active at higher light levels , are capable of colour vision. When image is formed at retina, light sensitive cells gets activated and generate electrical signal. These signals are sent to brain via optic nerve. Brain analyse these signals after which we perceive object as they are.

11. The vitreous body is the clear gel that fills the space between the lens and the retina of the eyeball of humans and other vertebrate The aqueous humor is a transparent, watery fluid similar to plasma, but containing low protein concentrations .

12. RODS - These are road-shaped cells of the retina that are sensitive to the intensity of light CONES- These are cone-shaped cells of the retina that are sensitive to the colours of light.

13. ACCOMMENDITION It is a the ability of eye lens due to which it can change its focal length so that image of object at various distance can be formed on the same retina.

14. RANGE OF NORMAL VISION - The distance between infinity and 25cm point is called the range of vision. LEAST DISTANCE OF DISTINCT VISION- The minimum distance from the eye, at which the eye can see the object clearly and distinctly without any stain is called the least distance of distinct vision. For a normal eye, is its value is 25cm. NEAR POINT- The nearest point from the eye, at which an object can be seen clearly by the eye is called its near point. The near point of a normal eye is at a distance of 25cm. FAR POINT- The farest point from the eye, at which an object can be seen clearly by the eye is called the far point of the eye. For a normal eye, the far point is at infinity.

15. PERSISTENC OF VISION- The phenomenon of the continuation of the impression of an image the object is cut off is called persistence of vision. The impression of the image remains on the retina for about ( 1/16th) of a second. Cinematography work on the principal of persistence of vision.

16. How Pupil Works ? Example: You would have observed that when you come out of the cinema hall after watching movie, in the bright sun light , your eyes get closed . And when you enter the hall from the bright light, you won'be able to see but after some time you would be able to see. Here the pupil of an eye provides a variable aperture, whose size is controlled by iris.

17. When the light is bright : Iris contracts the pupil, so that less light enters the eye . When the light is dim : Iris expand the pupil, so that more light enters the eye. Pupil opens completely, when iris is relaxed . Persistence of Vision : It is the time for which the sensation of an object continue in the eye. It is about 1/16th of a second. Power of Accommodation : The ability of eye lens to adjust its focal length with the help of ciliary muscles is called accommodation .

18. Eye lens becomes thin. Increases the focal length. Enable us to see distant object clearly Eye lens becomes thick Decreases the focal length. Enable us to see nearby object clearly

19. DEFECT OF VISION A normal eye can see objects clearly at any distance between 25cm and infinity from the eye. Sometimes, a human eye gradually lose its power of accommodation. Than we cannot see the object clearly our vision become defective. There are mainly four common defect of vision which can be corrected by the use of suitable eye glasses. There are Myopia or near sightedness Hypermetropia or far-sightedness Presbyopia Astigmatism

20. In this defect a person can see nearby object clearly but cannot see far off object clearly. Here, either the eyeball becomes too longer or the focal length of the eye lens become too short. It can be corrected by using a concave lens of suitable focal length. Focal length of the correcting lens = Distance of the far point from the eye.

21. LONG-DIGHTINED OR HYPERMETROPHIA In this defect a person can see the far off object clearly but he/she cannot see nearby object distinctly. Here, either the eyeball becomes too short or the focal length of the focal length of the focal length of the eye lens becomes too large. It can be corrected by using convex lens of suitable focal length. Focal length of correcting lens = yD /y-D Where y = Distance of the near point from the defective eye

22. PRESBYOPIA In this defect, a person in old age cannot read correctly due to the stiffening of the ciliary muscles and the decrease in flexibility of the eye lens .

23. ASTIGMATISM It is a defect of vision in which a person cannot simultaneously see both the horizontal and vertical views of an object with the same clarity . It is due to the irregular curvature of the cornea. It can be corrected by using a cylindrical lens.

24. COTARACT It is due to the development of hazy or opaque membrane over the eye lens which results in the decrease or loss of vision. It can be cured by surgery.

25. BLIND SPOT AND YELLOW SPOT In the region where the optic nerve enter the eyeball, there are not rods and cones. This region is totally insensitive to light and is called blind sport. Yellow spot has maximum concentration of light sensitive cells. It is situated in center of the retina.

26. COLOUR BLINDNESS A person who cannot distinguish between various colour but can see well otherwise, is said to be colour -blind. It is due to lack of some cones in retina of the eyes.

27. Example of an Ishihara color test plate. With properly configured computer displays, people with normal vision should see the number "74". Many people who are color blind see it as "21", and those with total color blindness may not see any numbers.

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