The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 11 Question Answer – The Human Eye are essential study tools for DAV public school students in Class 8.
DAV Class 8 Science Ch 11 Question Answer – The Human Eye
DAV Class 8 Science Ch 11 Solutions – The Human Eye
Something To Know
A. Fill in the blanks:
Question 1.
The ________ of the human eye determines the colour of a person’s eye.
Answer:
iris
Question 2.
The light sensitive screen of the human eye is called ________ .
Answer:
retina
Question 3.
The cone-shaped cells, on the retina, respond to the ________ .
Answer:
bright light
Question 4.
The defect of vision, in which eyeball becomes smaller than its normal size, is called ________ .
Answer:
hypermetropia
Question 5.
A ________ lens is used to correct myopia and a ________ lens is used to correct hypermetropia.
Answer:
convex, concave
B. Tick (✓) the correct option:
Question 1.
The change in focal length of an eye lens, to focus the image of the objects at varying distances, is done by the action of-
(a) Pupil
(b) Iris
(c) Retina
(d) Ciliary muscles
Answer:
(d) Ciliary muscles
Question 2.
The impression of the image formed on the retina of the human eye remains for about-
(a) 1/6th second
(b) 1/24th second
(c) 1/16th second
(d) 1/32nd second
Answer:
(a) 1/16th second
Question 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-
(a) Persistence of vision
(b) Far sightedness
(c) Near sightedness
(d) Power of accommodation
Answer:
(d) Power of accommodation
Question 4.
A near sighted person will face difficulty while reading-
(a) A book held in hands
(b) A text message on the mobile phone
(c) A street signboard across the street
(d) An e-mail, on the computer screen
Answer:
(c) A street signboard across the street
Question 5.
The defect of the human eye, in which the eye lens of a person gets progressively cloudy, resulting in blurred vision, is called-
(a) Near sightedness
(b) Far sightedness
(c) Cataract
(d) Night blindness
Answer:
(c) Cataract
Question 6.
The Braille system, for visually challenged persons, is a-
(a) Visual aid
(b) Auditory aid
(c) Tactual aid
(d) Electronic aid
Answer:
(a) Visual aid
C. State true or false for the following statements:
1. Iris is the muscular assembly that controls the size of the pupil.
Answer:
True
2. The eye lens forms a virtual, erect and diminished image of an object, on the retina.
Answer:
False
3. The retina of the human eye has a large number of cone-shaped and rod-shaped cells. The rods respond even in dim light.
Answer:
True
4. Cinematography makes use of the ‘persistence of vision’ of the human eye.
Answer:
True
5. For a normal human eye, the range of vision is from (nearly) 25 cm to infinity.
Answer:
True
6. Myopia is caused due to excessive curvature of the cornea.
Answer:
True
D. Answer the following questions in brief:
Question 1.
Name the part of the human eye which acts as a protective layer for the eye.
Answer:
Cornea.
Question 2.
The pupil appears black. Why?
Answer:
No light is reflected from pupil. Hence, the pupil appears black.
Question 3.
Differentiate between:
(a) Rod-shaped and cone-shaped cells
(b) Near point and far point of human eye
Answer:
(a) Rod-shaped cells get activated in dim light while cone-shaped cells get activated in bright light.
(b) Near point of human eye is 25 cm, while far point is infinity.
Question 4.
State the difference between
(a) Myopia and hypermetropia
(b) Tactual aid and auditory aid
Answer:
(a)
| Myopia | Hypermetropia |
| Difficulty in seeing distant objects. | Difficulty in seeing nearby objects. |
| Eyeball gets elongated. | Eyeball gets shortened. |
| Focal length of eye lens decreases. | Focal length of eye lens increases. |
| Corrected by concave lens. | Corrected by convex lens. |
(b) Tactual aid work on the touch receptors, i.e. skin, while auditory aids rely on our ability to hear. Braille system is an example of tactual aids. Audio caste is an example of auditory aid.
Question 5.
Define the terms
(a) Blind spot on the retina
(b) Persistence of vision of human eye
Answer:
(a) This is the spot where the optic nerve meets the retina. No photosensitive cells are present at this spot. So, no image is formed at this spot. Hence, this is called the blind spot.
(b) An image lasts on the retina for 1/16th second. This is called persistence of vision of human eye. Movies and animation work on this concept.
Question 6.
Give the meaning of terms
(a) Power of accommodation of the human eye
(b) Least distance of distinct vision
(c) Range of vision
Answer:
(a) The ability of the human eye to clearly focus on nearby as well as distant objects.
(b) The minimum distance up to which a normal eye can clearly see.
(c) The angle up to which a normal person can see with his/her eyes.
Question 7.
What happens to the image distance in the eye when the distance of the object from the eye increases?
Answer:
Irrespective of the object distance, the image distance in the eye is always same because image is always formed on the retina in a person with normal vision.
Question 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?
(b) What conclusion can be drawn about the ‘least distance of distinct vision’ of Sahil’s eye?
Answer:
(a) Beyond the retina
(b) The point of least distance of distinct vision in Sahil’s eye has increased significantly.
Question 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 needed to restore his proper vision.
Answer:
His eyeball has become elongated. He needs to use a concave lens to correct his vision defect.
E. Answer the following questions:
Question 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
Answer:
(a) Protects the eye.
(b) Adjusts the size of pupil.
(c) Allows light to enter the eye.
(d) Focuses the rays of light or retina.
(e) Controls the variations in focal length of eye lens.
(f) Image is formed on retina.
(g) Carries signals from the eye to the brain.
Question 2.
Explain how we perceive objects as they are?
Answer:
Light rays from an object enter our eyes and pass through the lens. The lens focuses these rays on the retina. An inverted, real and smaller image is formed on the retina. This stimulates photo receptors in the eye. The optic nerve picks signals from the eye and sends it to the brain. The brain interprets the signals and we are able to see an object.
Question 3.
State any five precautions that are needed to take care of our eyes.
Answer:
Following are five precautions to take care of our eyes:
- Do not read in dim light or too bright light.
- Do not keep your book too close to your eyes.
- Don’t look directly at the sun.
- Wash your eyes gently with clean water.
- Take balanced diet.
Question 4.
Explain how optical and non-optical aids help visually challenged persons to develop their skills to a higher level.
Answer:
Optical aids can help a partially visually impaired person. Optical aid can enhance the image or text so that the person can see them with some clarity. Non-optical aids rely on other sense organs like skin or ears. Tactual aids; like Braille rely on the sense of touch. Auditoiy aids; like audio cassettes rely on sense of hearing. These aids can help a visually impaired person to learn how to read and how to communicate effectively.
Question 5.
What is the Braille system? How does it help the visually challenged persons?
Answer:
This is a system of raised dots on paper by which a visually impaired person can read or write. There are 63 characters in the Braille system. It proved to be a boon for the visually challenged because books could be made in this system.
Value Based Questions
When Sumit joined his new school, he soon developed a veiy good understanding and friendship with Amit. Some miscreant students did not like this and succeeded in cre¬ating misunderstanding between them. Their Physics teacher observed their resulting ‘indifferent towards each other’ behaviour.
She called both of them to her room and told them about a (generally) age related defect of the eye. She explained to them that, in this defect, an opaque layer comes over the eye lens and blurs the vision. In the same way, she went on, misunderstanding can blur our thinking and weaken our mutual friendship and trust. She advised them to have a clear and open discussion with each other as this would help in clearing their misunderstandings and reviving their friendship.
Question 1.
What values are displayed by the class teacher in the above situation?
Answer:
The class teacher shows a concern for mental well being of her students.
Question 2.
Name the defect of vision caused by the formation of an opaque layer over the eye.
Answer:
Cataract
Question 3.
Have a group discussion on the importance of interpersonal understanding and trust in friendship.
Answer:
Do it yourself
Something To Do
Question 1.
Make a list of the students of Classes VII and VIII who use spectacles. Find out the nature of lens used and type of defect they have.
Answer:
Do it yourself
Question 2.
The human eye can play tricks on our brain. What we see is not always there, or is it so? There are several ‘optical illusions’ that demonstrate this point. Try to create figures/diagrams which give an impression of the optical illusions.
Answer:
Both lines are of equal length
Question 3.
Survey your neighbourhood. Is there any registered eye bank? Find out how this eye bank functions and helps visually impaired persons.
Answer:
Do it yourself
Question 4.
Make friends with some visually challenged students (It is a known fact that a person, with a sensory handicap, is gifted with some talent). Enquire from them how they master their disability and lead a fairly independent life.
Answer:
Do it yourself
Question 5.
Solve the puzzle with the help of clues given below:
| Across → | Down 4 ↓ |
| 1. Far sightedness | 1. Light sensitive cells. |
| 2. An opening through which light enters the eye. | 2. Defect corrected by an appropriate concave lens. |
| 3. The “white’ of the eye. | 3. The screen of the eye. |
| 4. Eye lens becomes opaque due to development of a membrane over it. | 4. System of raised dots on paper to enable the blind to read. |
| 5. Powerhouse of the eye. | 5. Sensory nerve cell which gets activated only in bright light. |
| 6. Dark coloured muscular diaphragm. | 6. Focuses the light rays on the retina. |
Answer:
| Across → | Down 4 ↓ |
| 1: hypermetropia | 1: photoreceptors |
| 2: pupil | 2: myopia |
| 3: cornea | 3: retina |
| 4: cataract | 4: Braille |
| 5: optic nerve | 5: cone |
| 6: iris | 6. eye lens |
DAV Class 8 Science Chapter 11 Solutions – The Human Eye Important Questions and Answers
I. Fill in the blanks:
1. Human eye is nearly ________ in shape.
Answer:
Spherical
2. The colour of the cornea is ________ .
Answer:
White
3. ________ controls the size of the pupil.
Answer:
Iris
4. The amount of light entering the eye is controlled by the ________ .
Answer:
Pupil
5. Eye lens becomes ________ in case of myopia.
Answer:
Enlarged
II. Tick the correct answer:
Question 1.
The image is formed on which part of the human eye?
(a) Cornea
(b) Iris
(c) Retina
(d) Pupil
Answer:
(c) Retina
Question 2.
Which of these decide the c-olour of a person’s eyes?
(a) Cornea
(b) Iris
(c) Pupil
(d) Retina
Answer:
(b) Iris
Question 3.
What is the maximum distance of clear vision for a normal eye?
(a) 10 m
(b) 100 m
(c) 1000 m
(d) Infinity
Answer:
(d) Infinity
Question 4.
Concave lens is prescribed in which of these conditions?
(a) Myopia
(b) Hyperrnetropia
(c) Cataract
(d) Glaucoma
Answer:
(a) Myopia
Question 5.
Convex lens is prescribed in which of these cases?
(a) Myopia
(b) Hyperrnetropia
(c) Cataract
(d) Glaucoma
Answer:
(b) Hyperrnetropia
III. Answer the following questions:
Question 1.
No image is formed at the blind spot in human eye. Why?
Answer:
No image is formed at the blind spot in human eye because there is no photosensitive cell at this spot.
Question 2.
What do you understand by persistence of vision?
Answer:
The image formed on the retina remains on the retina for about 1/ 16th of a second; even after the removal of the object. This phenomenon is called persistence of vision. Animations and movies take advantage of persistence of vision and we get an illusion of motion pictures.
Question 3.
What happens in case of myopia?
Answer:
In this eye defect, a person is unable to see distant objects clearly. In this condition, the light rays from a distant object converge before the retina; after passing through the eye lens. Due to this, a myopic person is unable to clearly see a distant object.
Question 4.
What happens in case of hyperrnetropia?
Answer:
In this eye defect, a person is unable to see nearby objects clearly. In this condition, the light rays from a nearby object converge behind the retina; after passing through the eye lens. Due to this, a hypermetropic person is unable to clearly see a nearby object.
Question 5.
Define a visually challenged person.
Answer:
A person with limited vision or with complete loss of vision is called a visually challenged person.
IV. Projects:
Question 1.
Make a model of human eye by using Styrofoam. Display it in your classroom.
Answer:
Do it yourself
Question 2.
Make a list of persons in your family and from your relatives who use spectacles.
Answer:
Do it yourself