Human Eye

 Structure and Function of the Human Eye

The human eye is a remarkable organ responsible for vision. A Sensitive sense organ. It acts like a camera, enable us to capture the colorful picture of the surroundings. It forms an inverted, real image on light sensitive surface Retina.

The Various parts of eye and their functions:

1. 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.

2. Eyeball: it is approximately spherical in shape, with a diameter of about 2.3cm.

3. Iris: It is a dark muscular diaphragm that controls the size of pupil. It is behind the cornea.

4. Pupil: It regulates and control the amount of light entering the eye. It is the black opening
      between aqueous humor & lens.

5. Crystalline eye lens Provide: 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.

6. 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 position.

7. Retina: Thin membrane with large no. of sensitive cells

8. When image formed at retina, light sensitive cells get activated and generate electrical signal. These signals are sent to brain via optic nerve. Brain analyses these signals after which we perceive object as they are.

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 entered the hall from the bright light, you won't be able to see and 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

a) When the light is bright: Iris contracts the pupil, so that less light enters the eye.

b) When the light is dim: Iris expand the pupil, so that more light enters the eye. Pupil open completely,
    when iris is relaxed.

Persistence of Vision: It is the time for which the sensation of an object continues in the eye. It is about
                                     1/16th of a second.

Power of Accommodation:

The ability of eye lens to adjust it focal length is called accommodation with the help of ciliary muscles.

Near Point of Eye:

It is 25 cm for normal eye. The minimum distance at which object can be seen most distinctly without strain.

Defects of Vision and Correction:

Myopia:

• Myopia, commonly known as nearsightedness.
  
  
• It can be managed and corrected through various methods.
  
  
• The far point for the myopic eye is nearer than infinity

   Occurs due to -

• Excessive curvature of the eye lens
  
  
• Elongation of the eyeball

The image of a distance object is formed in front of the retina and not on the retina. 

Defect is corrected by using Concave lens such that the lens will bring the image back to the retina.

Hypermetropia:

•      Hypermetropia, commonly known as farsightedness.
  
  
•     It is a refractive error of the eye where distant objects are seen more clearly than nearby objects.
  
  
•      It occurs when light entering the eye focuses behind the retina instead of directly on it.
  
  
•    This can happen due to the eyeball being too short or the cornea having insufficient curvature.

The image of a distant object is formed behind the retina. Defect is corrected by using a Convex lens.

Presbyopia:

• It is an age-related condition that affects the eye's ability to focus on nearby objects.
  
  
• It typically occurs around the age of 40 and progresses with age.
  
  
• Unlike myopia (nearsightedness) or Hypermetropia (farsightedness), which are caused by the shape of the eye, presbyopia is related to the natural aging process of the eye's lens.
  
  
• As people age, the lens becomes less flexible and loses its ability to change shape easily, making it challenging to focus on close-up objects, such as reading or using a smartphone.

Correction for presbyopia is often managed through:

• Reading Glasses: These are eyeglasses with lenses that have a higher optical power in the lower portion, specifically designed to aid near vision.
  
  
• Bifocal or Multifocal Lenses: These lenses have different optical powers in different areas, allowing individuals to see clearly at various distances (near, intermediate, and far).
  
  
• Progressive Lenses: These are similar to bifocals but have a more gradual transition between the different optical powers, providing a more natural visual experience. 

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