Chapter 11 | The Human Eye | Class- 8 DAV Science Solutions

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Chapter 11 The Human Eye

DAV Class- 8 Science The Human Eye Question and Answer

Something To Know

A. Fill in the blanks.

1. The iris of the human eye determines the colour of a person’s eye.

2. The light sensitive screen of the human eye is called retina .

3. The cone-shaped cells, on the retina, respond to the bright light.

4. The defect of vision, in which eyeball becomes smaller than its normal size, is called hypermetropia.

5. A concave lens is used to correct myopia and a convex lens is used to correct hypermetropia.

B. Write True or False for the following statements.

1. Iris is the muscular assembly that controls the size of the pupil.(1) True
2. The eye lens forms a virtual, erect and diminished image of an object, on the retina.(2) False
3. The retina of the human eye has a large number of cone-shaped and rod-shaped cells. The rods can respond even in dim light.(3) True
4. Cinematography makes use of the persistence of vision of the human eye.(4) True
5. For a normal human eye, the range of vision is from (nearly) 25 cm to infinity.(5) True
6. Myopia is caused due to excessive curvature of the cornea.(6) True

C. Tick (✓) the correct option.

1. The change in focal length of an eye lens, to focus the image of objects at varying distances, is brought about by the action of-

Answer: ciliary muscles

2. The impression of the image formed on the retina of the human eye remains there for about-

Ans: 1/16th second

3. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This ability of the eye is known as-

Ans: Power of accommodation

4. A near sighted person will face difficulty while reading-

Ans: A street signboard across the street.

5. The defect of the human eye, in which the eye lens of a person gets progressively cloudy, resulting in blurred vision, is called-

Ans: Cataract

6. The Braille system, for visually challenged persons, is a-

Ans: Tactual aid

D. Answer the following questions in brief.

1. Name the part of the human eye which acts as a protective layer for the eye.

Ans: Cornea protects outer layer of the eye.

2. The pupil appears black. Why?

Ans: pupil is a circular opening located in the center of the iris of the eye that controls the amount of light that enters the eye. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye.

3. Differentiate between:

(a) Rod-shaped and Cone-shaped cells

Ans:

Rod-shaped cellCone-shaped cells
These cells do not help us in colour vision.Cones-shaped cells get activated only in bright light conditions.
The rod-shaped cell gets activated in both bright and dim light conditions.These cells help us in colour vision.

(b) Near point and Far point of Human eye

Ans:

Near PointFar Point
The minimum distance at which objects can be seen easily is called Near Point. The Farthest Point at which objects can be seen easily is called Far Point.
The near point of distinct vision is about 25 cm in front of the eye. For a normal eye, the far point is at infinity

4. State the difference between:

(a) Myopia and Hypermetropia

Answer:

Myopia Hypermetropia
Myopia (or Near sightedness) is a defect of human eye in which a person is not able to see distant objects clearly. Hypermetropia (or far sightedness) is a defect of human eye in which a person is not able to see nearby objects clearly.
Myopia is corrected by using a Concave Lens of suitable powerHypermetropia is corrected by using a Convex Lens of suitable power

(b) Tactual aids and Auditory aids

Ans:

Tactual aidsAuditory aids

Tactual aids are non-optical sensory aids which involve the use of touch receptors.		Auditory aids are non-optical sensory aids which involve the sense of hearing, 
Braille system is an example of tactual aids.Tape recorders and Audio cassettes are examples of auditory aid.

5. Define the terms:

(a) Blind spot on the retina

Ans: A blind spot on the retina is an area of the retina that is light insensitive. This is spot where the optic nerve and the retina meet.

(b) Persistence of vision of human eye

Ans: The ability of an eye to continue to see the image of an object for a very short duration even after the object has disappeared from view is called persistence of vision.

6. Give the meaning of the terms:

(a) Power of accommodation of the human eye

Ans: Power of accommodation is the special property of the human eye to adjust the focal length to see the object clearly.

OR

The ability of the human eye to adjust itself such that it can see objects which are nearby and faraway is called Power of Accommodation.

(b) Least distance of distinct vision

Ans: The minimum distance, at which objects can be seen most distinctly, is called the least distance of distinct vision, or near point, of the eye.

(c) Range of vision

Ans: The distance, between the near point and far point of a normal eye, is called its range of vision.

7. What happens to the image distance in the eye when the distance of the object from the eye increases?

Ans: when the distance of the object from the eye increases the image distance in the eye will always be same. In every case, image is formed on the retina due to the power of accommodation.

8. When Sahil tried to read a book by keeping it at a distance of 25 cm, the text print appeared to be blurred.

(a) Is the image of the text getting formed before the retina, or beyond the retina of his eye?

Ans: The image formed is beyond the retina.

(b) What conclusion can be drawn about the least distance of distinct vision’ of Sahil’s eye?

Answer: The least distance of distinct vision of Sahil’s eye has increased as he is suffering from eye defect called hypermetropia.

9. A person, with a myopic eye, cannot see objects beyond 1.2 m distinctly. What can be said about the likely size of his eyeball? State the nature of the corrective lens that is needed to restore his proper vision?

Ans: The individual’s eyeball has been elongated. To correct his eyesight problem, he will have to use a concave lens.

E. Answer the following questions.

1. Draw a labelled sketch of the human eye. Explain the function of the following parts of the human eye:

(a) cornea (b) iris (c) pupil (d) eye lens (e) ciliary muscles (f) retina (g) optic nerves.

(a) cornea: Its main function is to act as a protective layer for the eye.

(b) iris: The iris is responsible for the characteristic colour of the eye of a person.

(c) pupil: Pupil is the self-adjustable aperture which regulates the amount of light entering the eye by adjusting its size.

(d) eye lens: The eye lens is a convex lens which focuses the rays of light on retina.

(e) ciliary muscles: These muscles help in changing the curvature and the focal length of the eye lens.

(f) retina: It is a light sensitive screen where image of the object is formed.

(g) optic nerves: Carries signals from the eye to the brain.

2. Explain how we perceive objects as they are.

Answer: The cornea and pupil allow light from an object to enter the eye. The eye lens converges these light beams on the retina to create a true, inverted, and reduced image of the object. When light lands on the retina’s light sensitive cells, they get activated and generate electrical signals. The optic nerves transmit these electric signals to the brain. The electrical signals are interpreted by the brain in such a way that we see an image that is vertical and the same size as the item.

3. State any five precautions that are needed to take care of our eyes.

Answer: Five precautions that are needed to take care of our eyes are:

● Wash your eyes regularly .
● Do not rub your eyes with dirty hands .
● Do not read in very dim and very bright light .
● Do not read in moving vehicles .
● Eat vegetables which are rich in vitamin A .

4. Explain how optical and non-optical aids help visually challenged persons to develop their skills to a higher level.

Answer: Optical and non-optical aids help visually challenged persons to develop their skills to a higher level in this way:

Optical aids: These include lenses and telescopic aids. While suitable lens combinations are used to rectify visual limitations, the telescopic aids can help to view class demonstrations and the blackboard.

Non-optical aids: These often include (1) tactual aids (using sense of touch; the visually impaired can learn to read and write by using a special touch system called the Braille. This system makes use of raised dots (put on a paper) and (2) auditory (using sense of hearing) aids include audio cassettes, tape recorders, audio CDs, audio books. Other electronic devices (like computer/calculator/closed circuit television) make use of software programmes that can transfer ‘text files’ into sound.

5. What is the Braille system? How does it help the visually challenged persons?

Answer: Braille is a coded language based on a logical system which was developed by Louis Braille for visually challenged person. There are Braille codes for common languages as well as, mathematical and scientific notations. Many languages (foreign/Indian) can be read using the braille system. Braille system has 63 dot patterns or characters. The braille characters make up the letters of alphabet, punctuation marks, etc. Dots/Characters are arranged in cells of two vertical rows of three dots each. Patterns of dots, used to represent letters of the English alphabet. These patterns, when embossed on Braille sheets, help visually challenged person to recognise words by touching.

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