Human Eye Function

Human Eye

The eye is the most important optical instrument that detects light and sends signals along the optic nerve to the brain.

It is an essential organ that gives us the ability to see. It allows  light sensing, vision and differentiating among different colors. It is far more delicate than even the finest photographic camera designed so far. One of the remarkable qualities of the eye is the ability to observe distinctly the objects situated at widely different distances from the eye. This attribute of the eye is called the accommodation of the eye.

Human Eye Diagram

The figure systematically shows the components of the eye.

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Parts of the Human Eye

The human eye has a nearly spherical shape of diameter about an inch having various parts in such a small sphere are described below:


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The front portion of the eye is more sharply curved and is covered by a transparent protective membrane called the cornea. It is this portion that is visible from outside.

Limbus: The limbus is the border of the cornea.

Sclera:  The outer white part of the eye.

Aqueous Humor

There is a space filled with a transparent watery fluid similar to plasma behind the cornea called the aqueous humor.

Crystalline Lens

The crystalline lens is held in the ciliary muscles and located behind the cornea. It is the name given to the natural lens that humans are born with.

Ciliary Muscles

Ciliary muscles are the small muscles that are attached to the crystalline lens that can make the lens change shape that allows the eyes to focus on near or far objects.


There is a muscular diaphragm in between the aqueous humor and the lens, called the iris. The iris is the colored part of the eye that we can see.


A small hole in an iris eye with a circular aperture is called the pupil. The pupil appears black as the light goes inside it has no chance of reflecting or coming back outside.


The light entering the eye forms an image on the retina that covers the inside of the near part of the eyeball. 

Vitreous Humor

The space between the lens and the retina is filled with a liquid called the vitreous humor.

Optic Nerve

It is also called the cranial nerve located at the backside of the eye.

Optic Axis

The line joining the center of the cornea to the center of the lens is called the optic axis.

Function of All Parts of The Eye

Ciliary Muscles

The shape i.e. the curvature and the focal length of the crystalline lens can be modified by the ciliary muscles.

Crystalline Lens

The image of an external object is formed by the crystalline lens on the retina.

The main function of a lens is to maintain the accommodation of the eye, maintain its own clarity, and refract the light.


The retina contains about 125 million receptors called the rods and cones that can sense the light intensity and color respectively and about one million optic-nerve fiber which transmits the information to the brain. The least sensitive part of the retina is called the blind spot.


The function of an iris is to adjust and allow a suitable quantity of light to the eye.


The front transparent hard outer part of the eye that protects the inner delicate parts of the eye.

What is The Function of Ciliary Muscles?

Case 1: When the eye is focused on the distant object

The ciliary muscles loosen up so that the focal length of the eye-lens has its maximum value which is equal to its distance from the retina.

Case 2: When the eye is focused on the closer object

The ciliary muscles become tense and the lens cannot curve enough to focus the image on the retina. The focal length of the lens decreases. Therefore, the image becomes blurred. However, the ciliary muscles adjust the focal length in such a way that the image is again formed on the retina and we see the image clearly. Such a process of adjusting the focal length to make an image visible clearly is called the accommodation.

Do you know?

  • The least distance of vision (d) for a normal eye is 25 cm (range of vision).

  • The human eye can distinguish about 10 million different colors approximately.

  • The human eye blinks at a mean of 4,200,000 times a year.

  • The eye is made of 10 million working parts and it weighs just 0.25-ounces.

  • The eyes remain the same size throughout life.

  • Eyes can process about 36,000 pieces of information in an hour.

FAQ (Frequently Asked Questions)

1. How is the amount of light entering the eye-controlled?

The purpose of the iris is to control the intensity of the light and then allow a suitable quantity of the light into the eye through the pupil where the pupil is the opening in the middle of the iris where light enters the eye.

2. Why has nature given us two eyes instead of one?

Our left eye observes more of the left side of the body and the right eye observes more of the right side of the body. The imprints of the two images mix in our brain to have an idea of solidity and the distance of the single object. However, we also get a 180° view of the surroundings. That is why nature has catered us with two eyes instead of one.

3. What part of the eye makes you see?

The Lens. The lens is composed of lucid, flexible tissue that is positioned directly behind the iris and the pupil. It is the second part of your eye, following the cornea, that helps to focus the light and images on your retina.