11. Human Eye and Colourful World

Human Eyes: 

The human eye is one of the most valuable and sensitive sense organs. It is spherical in shape and it works like a camera. We can able to see this colourful world. It has a crystalic lens which forms an image on an another light sensitive part of eyes called retina. The light passes into eyes through a thin membrane called cornea. There is an another structure behind the cornea called iris. This controls the size of pupil. Pupil is a dark muscular diaphragm (hole like structure) that controls the amount of  light entering into eye. The light-sensitive cells of retina get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. 

The Part of Eye:

(1) Cornea: Cornea is a thin membrane of outer and blackish part of eye through which light enters into the eye. It makes outer covering of anterior portion of eye.  

(2) Iris: Iris is a dark muscular diaphragm that controls the size of the pupil. it is situated just behind the cornea. 

(3) Pupil: The pupil regulates and controls the amount of light entering into the eye. 

(4) Ciliary Muscle: It controls the curvature of eye lens by relaxation and contraction causing changing in focal length of lens. 

(5) Vitreous Humour: The remain eye ball fulled with fluid like substances called Vitreous Humour. Which located between eye lens and retina. It gives the spherical shape to the eye ball. 

(6) Retina: The retina is a delicate membrane having enormous number of light-sensitive cells. It works as screen to form image by eye lens.  

There are two types of light-sensitive cells present on Retina

(i) Cylindrical: 

(ii) Conical: 

(7) Optic Nerve: It is a main nerve that join eye to brain. Signals passes to the brain via optic nerve. 

(8) Crystalic Lens: This is a convex lens which is composed of a fibrous, jelly like material. 

The nature of Image formed by eye lens: Real and inverted. 

The curvature of eye lens is modified to some extent by the ciliary muscles. This change in the curvature of eye lens can thus change its focal length. 

Increasing in focal length: When the ciliary muscles are relaxed, the lens becomes thin. This causes its focal length increases. This enables us to see distant objects clearly. 

Decreasing in focal length: When we looking at closer objects, the ciliary muscles contract. This increases the curvvature of the eye lens. This causes eye lens thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly. 

Power Of Accommodation:

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

Near Point : 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. For a young adult with normal vision, the near point is about  25 cm.

Far Point: The farthest point upto which the eye can see objects clearly is
called the far point of the eye. It is infinity for a normal eye.

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