Human Eye and defects of vision, practice problems, FAQs

You may see people wearing glasses around you, maybe you also wear one. But why do they wear it? For seeing an object, one part of our eye focuses the image to the back part(called retina) of the eye where it converts into a signal and sends it to the brain and the image is perceived. Excessive strain on the eye or aging causes the defect and image is not formed on retina, but in front or back of retina. To correct this defect we have to wear glasses. Lets understand more about the Eye and its defects!

Table of content

• Human eye
• Far Point and Near Point
• Apparent size 
• Accommodation
• Defects of human Eye and Correction
• Practice problems
• FAQs

Human eye

Human eye is an organ of the human body which is used for vision. It is located in the human skull. Approximate size of a human eye is 2.5 cm in diameter. The important parts of eye are:

1. Cornea- The transparent portion of the eye that allows the light to enter our eye is known as the ‘cornea’. It is made up of transparent tissue. It covers the various parts like the pupil, anterior chamber, and the iris. It refracts light and accounts for two-thirds of the eye’s total power. The refractive power of cornea is approximately 43 dioptres.

2. Iris- Iris is made up of contracting and relaxing muscles in the eye. It controls the size of the pupil and the light reaching the retina. Iris defines the color of your eyes.

3. Pupil- Pupil is placed in the center of the iris and allows light to reach the retina. Its size is controlled by the iris. It appears in black color so the eye tissues absorb or diffusely reflect the light entering the pupil.

4. Lens- Biconvex lens of transparent structure present in the eye behind the pupil. The lens refracts the light, so it can focus on the retina. The lens is capable of changing the focal distance by changing its size. When Eye sees the object at infinity it is said to be in its relax mode.

5. Retina- After refracting the light from the lens it comes on a light-sensitive tissue called retina, which is in the inner coat of the eye that sends electrical signals after converting them from light to the brain for processing. Two light-sensitive cells are present in the retina - one is Rods and other cones. Rods for night-time vision and cones to see the colors. 

6. Optic Nerves- The optic nerve is used to send electrical impulses from the retina to the brain.

Now lets see the defects in the human eye.

Far Point and Near Point

Distance of the farthest object that can be seen clearly by naked eye is called the Far point of eye. For the normal human eye it is at infinity.

Closest distance of an object at which it can be seen clearly by the naked eye is known as the Near point of eye. It is denoted by ‘D’ and for a normal human eye it is 25 cm.

Apparent size 

The size of an object as sensed by us depends on the size of the image formed on the retina. As shown in figure the object which is near to the eye forms a bigger size of image than the object which is away from the eye.

The  angle subtended by the object on eye is called Visual angle and defines the apparent size of the object.

Where h_o= height of object 

u= position of object

As seen by naked eye, apparent size is maximum for the object at a near point . 

Apparent size of an object can be increased by increasing using an optical instrument .

Accommodation

The ciliary muscles adjust the focal length of the lens in order to produce a sharp image on the retina. This process of adjusting focal length is called accommodation.

Defects of human Eye and Correction

Defect in the eye means that the proper image is not formed on the retina. Here are some common defects of eye:

1. Myopia or Near-sightedness- It is a common vision condition in which you can see objects nearby clearly, but objects farther away are blurry. It occurs when the shape of the human eye causes light rays to bend (refract) incorrectly, focusing images in front of the retina instead of on the retina.The far point for this eye is closer than infinity.

For the correction of this defect we wear contact glasses made of concave lenses.

It diversifies the light rays so they can focus on the retina.

2. Hypermetropia or Far-sightedness- It is a condition of the eye where distant objects are seen clearly but nearby objects appear blurred. It is due to incoming light being focused behind, instead of on the retina. The near point for this eye increases. 

The correction of hypermetropia can be done by wearing glasses made of convex lens. It will converge the light ray and focus on the retina.

3. Astigmatism- In Astigmatism the light rays entering the eye do not focus evenly to a single focal point on the retina but instead are scattered. Some light rays focus on the retina and some focus in front of or behind it. This is because of non-uniform curvature of the cornea; resulting in a distorted or blurry vision at any distance.

Correction of astigmatism can be done by using a special spherical cylindrical lens.

Video explanation of various

Practice problems

Q1. A person is able to see objects clearly only when they lie between 50 cm and 400 cm from his eye. In order to change distinct vision to infinity, the type and power of the correcting lens will be? 
Answer: For increasing the distinct vision to infinity the image by the lens should be at 400 cm.

Hence

 v=-400

Using lens formula 

f=-400 cm

Power of lens 

- ve sign shows that the lens is concave. 

Q2. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect?
Answer: It is given that the near point of defective eye is 1 m. For a normal eye it is 25 cm. 

Hence u=−25 cm. 

v=−1 m=−100 cm. 

Using lens formula 

f=33.33 cm

Power of lens 

+ ve sign shows that the lens is convex. 

Q3.A person cannot see objects beyond 80 cm, whereas a normal person can easily see an object distant 160 cm. What is the focal length and nature of the lens used to rectify this defect?
Answer: Given u=-160

 v=-80

Using lens formula 

f=-160 cm

Q4. A person can not see nearer to the point 40 cm. Which lens to be used for its correction and what should be the power?
Answer: Given the near point of a hypermetropic eye 40 cm

Hence u=−25 cm. 

v=−40 cm. 

Using lens formula 

Power of lens 

+ ve sign shows that the lens is convex.

FAQs

Question 1. What is the cause for Myopia or short- sightedness?
Answer: It happens due to the eyeball being too long and the high converging power of the eye.

Question 2. What is the advantage of having two eyes?
Answer: When one eye is open, a human being can see a horizontal field of view of about 150° but with two eyes open, the field of view is 180^°. Thus with two eyes we can see a wider field of view.

Question 3. What is the nature of the image formed on the retina?
Answer: Inverted image is formed on the retina, but our brain helps in interpreting it as an erect image.

Question 3. What is night blindness?
Answer: Night Blindness is a retina problem which prevents you from seeing well at night or in poor lighting.