[Physics Class Notes] on Human Eye Function Pdf for Exam

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 allow the eyes to focus on near or far objects.

• Iris
  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.

• Pupil
  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.

• Retina
  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 vitreous humor.

• Optic Nerve
  The optic nerve is a part of the human eye that is located behind the eye. It is the second cranial nerve that helps in carrying the vision of the eye and reflects it to the human brain. It transfers the electric impulses from the retina and projects them in the cortex of the human brain. It reaches the brain through the optic disk and that particular point is called the blind spot.

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

 

Functions 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.

• Retina
  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.

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

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

 

Myopia and Hypermetropia

• Myopia is a condition of damage to vision. This condition is caused when the image of the object we see forms behind the retina as the curvature of the retina increases and it is difficult to see distant objects. This condition is also known as short-sightedness and can be rectified by using diverging lenses recommended by an optician.

• Hypermetropia is also known as long-sightedness is also caused when the vision of our eye is affected. In this condition, the image of the object we see forms in front of the retina, and this is caused when the structure of the eye is affected. One can not see objects placed closer to the eye in this condition as the focal length increases for such people. Some of the structural defects which might lead to this case are small eyeball size, non-circular lenses, weak ciliary muscles, defective blood vessels in the retina, change in the refractive index of the eye when the position of the lens is affected.

 

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.

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