Physics notes: The Human Eye For ALP Stage-2 exam 2018

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Human Eye’ is the organ of vision of the human body that enables us to see.  The human eye(s) are located in the specialized sockets carved out in the human skull. Each human eye sizes for approximately 2.5 cm in diameter. The eye lens forms an inverted real image of the object on the retina.

The main parts of a human eye are:

RETINA - The retina is a delicate membrane having enormous number of light-sensitive cells.

CORNEA -  Light enters the eye through a thin membrane called the cornea.It is the eye’s outermost layer. It is the clear, dome-­shaped surface that covers the front of the eye. It plays an important role in focusing your vision.

PUPIL - The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye. The pupil regulates and controls the amount of light entering the eye.

IRIS - It is a dark muscular diaphragm that controls the size of the pupil and thus the amount of light reaching the retina.

CILIARY MUSCLE - The ciliary muscle is a ring of smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye, not the size of the pupil. 

The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.

When the light is very bright, the iris contracts the pupil to allow less light to enter the eye. However, in dim light the iris expands the pupil to allow more light to enter the eye. Thus, the pupil opens completely through the relaxation of the iris.

A human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes. 

Near point or Least distance of distant vision:
- The minimum distance at which objects can be seen most distinctively without strain.
- For normal adult eye, its value is 25cm
- Range of human vision- 25cm to infinity.

ACCOMMODATION
The ability of the eye lens to adjust its focal length is called accommodation. Focal length can be changed with the help of ciliary muscles.
- Focal length increases when ciliary muscles get relaxed and lens get thin.
- Focal length decreases when ciliary muscles get contract and lens get thick.

CATARACT- The condition of partial or complete loss of vision is called cataract. It is caused due to the membrane growth over the eye lens. The crystalline lens becomes milky or cloudy in this condition.

DEFECTS OF VISION AND THEIR CORRECTION

1. MYOPIA or short sightedness : A person suffering from myopia can see the near objects clearly while far objects are not clear.

Causes :
•Elongation of eye ball along the axis.
•Shortening of focal length of eye lens.
•Over stretching of ciliary muscles beyond the elastic limit. 

Correction: It is done by using concave lens of appropriate power. The concave lens placed in front of the eye forms a virtual image of distant object at far point of the myopic eye.




2. HYPEROPIA or HYPERMETROPIA or longsightedness : A person suffering from hypermetropia can see the distant objects clearly but not the near objects.

Causes:
•Shortening of eye ball along the axis.
•Increase in the focal length of eye lens.
•Stiffening of ciliary muscles.

Correction- Use of convex lens of suitable power can correct the defect.




3. PRESBYOPIA : This defect is generally found in old people. Due to stiffening of ciliary muscles, eye looses much of its accommodating power. As a result distant as well as nearby objects can not be seen.The near point of the old person having presbyopia gradually recedes and becomes much more than 25cm away.

Causes:
- Gradual weakening of ciliary muscles
- Diminishing flexibility of eye lens

Correction:
- By wearing bifocal glasses or Progressive Addition Lenses(PALs) wherein the upper portion of the lens contains concave lens and lower portion contains convex lens.

4. ASTIGMATISM : Astigmatism is a defect wherein the light rays entering the eye do not focus light evenly to a single focal point on the retina but instead scatter away. The light rays scatter in a way where some focus on the retina and some focus in front or behind it. 

Causes:
- non-uniform curvature of the cornea; resulting in a distorted or blurry vision at any distance.

Correction of astigmatism:
-It can happen by using a special spherical cylindrical lens.


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