Human eye:

         Why do we study human eye in the Physics? If you have the same question, here is the answer. Study of light is important because  it the biggest source of information for 99.99% people in the world. Also in the human eye there a lens inside and therefore the study becomes interesting.

      I have given short notes on whole content of the human eye. Do read and do not forget to add your feedback

Human eye consist of

1. Structure and function of human eye.
2. Power of accommodation
3. Eye defects such as myopia, Hypermetropia and Presbyopia.

 Structure and function of human eye

Cornea - Most of the light get refracted at cornea. it also acts as the protective layer. Light enters the eye through this part

Iris - Iris is a dark muscular diaphragm that controls the size of the pupil. 

Pupil - The pupil regulates and controls the amount of light entering the eye.

Crystalline lens - It focus the image on the retina

Ciliary muscles - this are stretchable muscles which controls the focal length of the crystalline lens.

Retina - It acts as the screen. The retina is a delicate membrane having enormDefectsumber of light sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals.

Optical nerve - Signals created by retina is sent to the brain through the optical nerve. 

Brain - it processes the image as the image formed on the retina is the inverted image.

Power of the accommodation:

"The ability of the eye lens to adjust its focal length is called accommodation or the power of accommodation. "

      When we look at near by objects, focal length of the lens decreases and the ciliary muscles get elongated and when we look at distant ( farther) objects focal length of the lens increases and the ciliary muscles get relaxed. and this is how ciliary muscles helps us to see near by as well distant objects clearly.

Near point (least distance of distinct vision)-  The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye. It is 25 cm for a normal eye.

Far point (maximum distance of distinct vision - The farthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.

Cataract

Sometimes, the crystalline lens of people at old age 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.

Eye Defects

Myopia :

• "A person with myopia can see nearby objects clearly but cannot see distant objects distinctly"
• Myopia is also known as near-sightedness or short-sightedness.
• Near point- 25 cm
• Far point- nearer than infinity
• The image of a distant object is formed in front of the retina.
• Causes- 1. excessive curvature of the eye lens

                            2. elongation of the eyeball.

• This defect can be corrected by using a concave lens of suitable power.

Hypermetropia:

• "A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly."
• It is also known as far-sightedness.
• Near point is farther away from 25 cm.
• Far point is infinity.
• The image of nearer object is formed behind the retina.
• Causes- 1. the focal length of the eye lens is too long, 

                          2. the eyeball has become too small.

• This defect can be corrected by using a convex lens of appropriate power.

Presbyopia:

• "The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. This defect is called Presbyopia."
• Causes- 1. the gradual weakening of the ciliary muscles, 

                           2. diminishing flexibility of the eye lens.

• Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses.
• Bi-focal lens:  A bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens and the lower part is a convex lens.