Our Eyes – Characteristics of a Human Eye

Our Eyes 

 Key Concepts

• The human eye
• Parts of an eye
• Taking care of eye
• An alternative to visually impaired people

Introduction

When light getting reflected from the surrounding objects enters our eyes, we see the objects. Therefore, our eyes are essential for us to see everything around us. In this section we will learn about the human eye, its structure and function of its various parts. We will also look at the ways to take care of our eyes and the alternative for visually impaired people. 

Explanation

The human Eye: 

The following are the characteristics of a human eye: 

• A human eye has a roughly spherical shape. 
• The outer white coat called sclera is tough in order to prevent it from injuries and accidents. 
• The transparent part at the front is called the Cornea. 
• There are colored muscular structures behind the cornea called the iris. The color of the eyes of a person is the color of the iris of their eyes. 
• The iris has a small circular opening called the pupil, whose size is controlled by the iris. 

• When light is shone onto a person’s eye, the iris relaxes and the pupil size decreases. 
• This is done to decrease the amount of light entering the eyes. 
• Under dim light, the iris contracts to increase the pupil size so as to increase the amount of light entering the eyes. 
• The amount of light entering the eyes is controlled by the iris by changing the pupil size. 
• There lies a lens behind the pupil of the eye. This lens is thicker in the middle and narrows out towards the edges. 

• Such lenses, are called convex lenses, as both their surfaces are convex. 
• This lens focuses light entering the eyes on to a layer, which lies at the back of the eye, called the retina. 
• As a result, a real and inverted image of an object is formed on the retina by the lens. 
• The retina consists of nerve cells which are sensitive to the images formed on it. They are basically of two types—Rods and Cones. 
• Rod cells, that are sensitive to dim light. 

• Cone cells, that are sensitive to bright light and also sense color of the objects. 
• The sensations felt by these cells in the retina are then transmitted to the brain by the optic nerve.  
• There are no rods and cones at the place where the optic nerve and the retina meet. This spot is called the blind spot. No vision is possible at this spot. 
• The signals from the retina transmitted to the brain, is interpreted by the brain to show an upright image of the objects around. 
• An image formed on the retina does not vanish immediately, it persists there for about 1/16^th of a second.  

• Therefore, if still images of a moving object are formed on the retina of the eye at a rate faster than 16 images per second, the object is perceived to be moving. 
• The movies usually flash 24 pictures per second. Thus, we perceive it as a moving picture. 
• Eyes can look at a nearby object and also at an object lying far away.  
• This is because the eye lens is capable of changing its size, which is done by the ciliary muscles, that hold the eye lens in place. 
• However, there is a minimum distance at which a human eye can see things clearly, which is 25 cm for a normal eye. This distance is called the least distance of distinct vision 

Taking care of the eyes: 

It is very important to take care of the eyes. Following are some of the measures by which one can take care of their eyes. 

1. Visit a doctor if there is any discomfort in the eyes. 

2. Use corrective lenses only when prescribed by a doctor. 

3. Too little or too much light, both are not good for the eyes. Too little light might strain the eyes and too much light can injure the retina. 

4. If some dust enters the eyes, never rub your eyes. Wash your eyes with clean water instead. If there is no improvement, visit a doctor. 

5. While reading something, keep the book at a minimum distance of 25 cm (least distance of distinct vision) from the eyes. Do not take it too far as well. 

6. The deficiency of vitamin A also causes eye problems. Thus, it is advisable to take food items which are rich in vitamin A such as broccoli, raw carrots, eggs, butter, papaya, mango, spinach etc. 

An alternative to visually impaired people: 

• The people who have limited vision or no vision are said to be visually impaired. Some people are visually impaired since their birth and others lose their vision to a disease or an injury. 
• The most popular alternative to visually impaired people is the Braille system, which was developed by Louis Braille, who himself was a visually impaired person. 
• The Braille system was adopted in the year 1932. 
• The Braille system has 63 dot patterns or characters, each of which represents a letter, a combination of letters, a common word or a grammatical sign. 
• Dots are arranged in cells of two vertical rows of three dots each, as shown in the adjacent figure. 

• These patterns are embossed on Braille sheets, which help the visually impaired people to recognize words by touching. 
• The dots are raised slightly as shown in the adjacent figure to make it easier to touch. 

Summary

1. Our eyes are essential for us to see the objects around us.
2. The human eye is roughly spherical in shape with a strong outer layer called sclera,
   whose front transparent part is called the cornea.
3. The iris of the eye are muscles that lie behind the cornea and are responsible for the
   color of the eyes of a person. They control the amount of light entering into the eyes by
   changing the size of the pupil.
4. The iris relaxes in bright light and contracts in dim light, leading to a decrease and an
   increase in the size of the pupil.
5. Behind the iris, there lies a lens held by the ciliary muscles. The lens is thicker at the
   middle and narrows towards the edges.
6. The ciliary muscles help to change the size of the lens whenever required, because of
   which a person can see both the nearby and the distant objects clearly
7. The lens focuses the light entering the eye onto the retina, which lies at the back of the
   eye. As a result, a real and inverted image of an object is formed on the retina.
8. The retina consists of rod and cone cells, which help to recognize an object in dim and
   bright light respectively. The cone cells are responsible for identifying the colors.
9. The signals from the retina are transmitted to the brain through the optic nerve.
10. The junction of the retina and the optic nerve has no rods or cones. Hence no vision is
    possible there. This area is known as the blind spot.
11. The signals are interpreted by the brain to show us the final upright image of the object.
12. An image formed on the retina does not vanish immediately, it persists there for about
    1/16th of a second. If still images of a moving object are formed on the retina of the eye
    at a rate faster than 16 images per second, the object is perceived to be moving.
13. The minimum distance at which a human eye can see things clearly, which is 25 cm for
    a normal eye, called the least distance of distinct vision.
14. The people who have limited vision or no vision are said to be visually impaired.
15. Braille system can be used by visually impaired people to read.