Human Eye- Parts of Human Eye

The Human Eye 

Introduction

In this session we are going to look at the way in which various parts of the human eye work together for us to see an object the way we do. Also, we are able to see objects located at various distances with the same clarity. This is because of a special ability of the eye lens, which is going to be covered in this session. 

Explanation

Parts of the human eye: 

The human eye is almost spherical in shape. The outer white coat called sclera is tough in order to prevent it from injuries and accidents. 

Cornea is a transparent layer in front of the eye, which helps the eye focus light for clear vision. Most of the refraction of light occurs here. The eye lens merely provides finer adjustments to focus the light rays on the retina. 

Iris is the colored muscular structure behind the cornea, which determines the color of the eyes and also controls the size of the pupil. 

The pupil is a small opening in the iris whose size is controlled by the iris. The size of the pupil decides the amount of light entering the eyes. The pupil size decreases when there is enough light available. The pupil size increases when there is not enough light available. The amount of light entering the eyes is controlled by the iris by changing the pupil size. 

The eye lens is a transparent and elastic structure, located behind the pupil of the eye. It has a convex shape. It adjusts itself to focus the objects located at various distances at the retina. 

The ciliary muscles change the shape of the lens in order for it to focus the light coming from objects located at various distances at the retina. 

The eye lens forms a real and inverted image of any object on the retina. 

The retina is a delicate layer lining the inside of the back of the eye. It has enormous number of light-sensitive cells, which get activated upon exposure to light photons and generate electrical signals. 

These signals are sent to the brain via the optic nerves. The brain interprets these signals and processes the information so that we perceive objects as they are. 

Power of accommodation: 

The eye lens is composed of a fibrous, jelly-like material, which can change its focal length to some extent by the help of the ciliary muscles. This changes the curvature and hence the focal length of the lens. 

When the ciliary muscles are relaxed, the lens becomes thin, decreasing the curvature of the lens and increasing its focal length. This enables a person to see the distant objects clearly. 

When the ciliary muscles contract, the lens becomes thicker, increasing the curvature of the lens and decreasing its focal length. This enables a person to see the nearby objects clearly. 

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

Near point and Far point: 

The focal length of the eye lens cannot be decreased below a certain limit. 

To see an object clearly and comfortably, it needs to be placed at a minimum distance of 25 cm. 

This minimum distance is called the least distance of distinct vision. Also called the near point of the eye. 

The near point of a normal eye is 25 cm. 

The farthest point up to which the eyes can see clearly is called the far point of the eye. It is infinity for a normal eye. 

Therefore, a normal eye can see the objects clearly between 25 cm to infinity. 

Questions and Solutions: 

1. Are you able to read a book placed too close to your eyes? Give reasons for your answer. 

Answer: 

We will not be able to read a book placed too close to our eyes. This is because the eye lens cannot reduce its focal length so as to form a clear image of the text on the retina. 

2. In which part of the eye does the maximum refraction of light takes place? 

1. The eye lens 

2. The cornea 

3. The sclera 

4. The vitreous humour 

Answer: The Cornea.  

Summary

1. The eyeball is approximately spherical in shape.
2. It has several parts, each with a specific function such as the outer covering comprises of
   the sclera and the cornea, the amount of light entering the eye is regulated by the iris and the pupil, the light is focused onto the retina by the cornea, the eye lens and the ciliary muscles.
3. The eye lens forms a real and inverted image of an object placed in front of the eyes on
   the retina.
4. The ability of the eye lens to adjust its focal length is called the accommodation.
5. The minimum distance from which an object can be seen clearly without strain in the
   eyes is called the least distance of distinct vision or the near point of the eye.
6. The farthest point up to which the eyes can see clearly is called the far point of the eye. It
   is infinity for a normal eye.
7. Therefore, a normal eye can see the objects clearly between 25 cm to infinity.