DAV Class 8 Science Chapter 11 Question Answers – Human Eye Solutions
Right now your eyes are doing something amazing. Light from this screen enters them, gets focused into a tiny picture, and your brain turns it into words you can read, all in a fraction of a second. Chapter 11 of the DAV Class 8 Science book explains how the human eye works, what can go wrong with it, and how people who cannot see well learn and work. This post gives easy answers to every question in the chapter, with a short reason behind each one, so you can understand the answer and write it in your own words.
The chapter in one minute
Light from an object enters the eye through the cornea, a clear front layer, and then passes through the pupil, the opening in the middle of the eye. The iris, a ring of muscle around the pupil, makes the pupil bigger in dim light and smaller in bright light. Behind the pupil is the eye lens, a convex lens that focuses the light on the retina, the light-sensitive screen at the back of the eye. The retina turns the picture into electrical signals, and the optic nerve carries them to the brain.
The retina has two kinds of cells. Rod cells work in dim light but do not see colour. Cone cells work in bright light and let us see colours. The eye can see objects at different distances because the ciliary muscles change the shape of the lens, and this ability is called the power of accommodation.
A normal eye sees clearly from about 25 cm, the near point, up to infinity, the far point. Two common defects are myopia (near-sightedness) and hypermetropia (far-sightedness), and both can be corrected with spectacles. People with visual challenges use optical aids like special lenses and non-optical aids like the Braille system.
A. Fill in the blanks
1. The iris decides the colour of a person’s eyes.
2. The light-sensitive screen of the eye is called the retina.
3. The cone-shaped cells on the retina respond to bright light.
4. The defect in which the eyeball becomes smaller than normal is called hypermetropia.
5. A concave lens corrects myopia, and a convex lens corrects hypermetropia.
B. True or false
| Statement | Answer | Why |
|---|---|---|
| 1. The iris is the muscle that controls the size of the pupil. | True | It widens or narrows the pupil to control how much light enters. |
| 2. The eye lens forms a virtual, erect, diminished image on the retina. | False | The image is real, inverted and diminished. The brain turns it the right way up. |
| 3. Rods respond even in dim light. | True | Cones need bright light, and rods work in dim light. |
| 4. Cinematography uses the persistence of vision. | True | Still pictures shown quickly one after another look like movement. |
| 5. A normal eye sees from about 25 cm to infinity. | True | This is the range of vision of a healthy eye. |
| 6. Myopia can be caused by excessive curvature of the cornea. | True | An overly curved cornea (or an elongated eyeball) focuses light in front of the retina. |
C. Choose the correct option
1. The change in the focal length of the eye lens is brought about by the ciliary muscles.
2. The image on the retina stays there for about 1/16th of a second.
3. The ability of the eye to focus objects at different distances is called the power of accommodation.
4. A near-sighted person will find it difficult to read a street signboard across the street, because distant objects look blurred.
5. The defect in which the eye lens gradually turns cloudy and vision becomes blurred is cataract.
6. The Braille system is a tactual aid, because it is read by touch.
D. Short answer questions
1. Which part of the eye acts as a protective layer?
The cornea. It is the clear front layer that covers and protects the eye, and it also helps bend light into the eye.
2. Why does the pupil look black?
The pupil is an opening. The light that goes in is absorbed inside the eye, and almost none of it comes back out, so the pupil looks black.
3. Differentiate between the following.
(a) Rod cells and cone cells
| Rod cells | Cone cells |
|---|---|
| Work in both dim and bright light. | Work only in bright light. |
| Do not help us see colours. | Help us see colours. |
(b) Near point and far point
| Near point | Far point |
|---|---|
| The closest point at which the eye can see an object clearly without strain. | The farthest point up to which the eye can see an object clearly. |
| About 25 cm for a normal eye. | At infinity for a normal eye. |
4. State the difference between the following.
(a) Myopia and hypermetropia
| Myopia (near-sightedness) | Hypermetropia (far-sightedness) |
|---|---|
| Nearby objects are clear, but distant objects look blurred. | Distant objects are clear, but nearby objects look blurred. |
| The image forms in front of the retina. | The image forms behind the retina. |
| The eyeball is too long, or the lens is too curved. | The eyeball is too short, or the lens is too flat. |
| Corrected with a concave lens. | Corrected with a convex lens. |
(b) Tactual aids and auditory aids
| Tactual aids | Auditory aids |
|---|---|
| Work through the sense of touch. | Work through the sense of hearing. |
| Example: the Braille system. | Example: audio cassettes and audio books. |
5. Define the following terms.
(a) Blind spot: It is the spot on the retina where the optic nerve joins it. There are no light-sensitive cells there, so it cannot sense light.
(b) Persistence of vision: It is the ability of the eye to keep seeing an image for a very short time (about 1/16th of a second) after the object has gone out of view.
6. Explain the following terms.
(a) Power of accommodation: It is the ability of the eye to change the focal length of its lens so that it can see objects clearly at different distances.
(b) Least distance of distinct vision: It is the shortest distance at which an object can be seen clearly without strain. It is also called the near point, and it is about 25 cm for a normal eye.
(c) Range of vision: It is the distance between the near point and the far point of the eye.
7. What happens to the image distance in the eye when the object moves farther away?
The image distance does not change. The image is always formed on the retina, which is at a fixed distance from the lens. The ciliary muscles change the focal length of the lens, so the image stays on the retina however far the object is.
8. Sahil finds the print blurred when he holds a book 25 cm away.
(a) The image is formed behind (beyond) the retina.
(b) His least distance of distinct vision has increased. He cannot see nearby objects clearly, which shows he has hypermetropia.
9. A myopic person cannot see objects beyond 1.2 m clearly. What can be said about his eyeball? Which lens will correct his vision?
His eyeball has become elongated (too long), so the image of distant objects forms in front of the retina. He needs a concave lens to correct this.
E. Long answer questions
1. Draw a labelled sketch of the human eye and explain the function of each part.
In your diagram, draw a round eyeball and label the cornea, the iris, the pupil, the eye lens, the ciliary muscles, the retina and the optic nerve. Here is what each part does:
- Cornea: It is the clear outer layer that protects the eye and lets light in.
- Iris: It is the coloured part of the eye, and it decides the colour of a person’s eyes.
- Pupil: It is the opening in the middle of the iris. It becomes bigger or smaller to control how much light enters the eye.
- Eye lens: It is a convex lens that focuses light on the retina.
- Ciliary muscles: They change the curvature and focal length of the lens.
- Retina: It is the light-sensitive screen where the image is formed.
- Optic nerve: It carries signals from the retina to the brain.
2. Explain how we see objects as they really are.
Light from an object enters the eye through the cornea and the pupil. The eye lens bends the light and forms a real, inverted and smaller image on the retina. The light-sensitive cells in the retina produce electrical signals, and the optic nerve carries them to the brain. The brain reads these signals and turns the image the right way up, so we see the object upright and in its real size.
3. State any five precautions to take care of our eyes.
- Wash your eyes gently with clean water often.
- Eat a balanced diet with plenty of fruits and vegetables.
- Never look directly at the sun or any very bright light.
- Do not stare at a computer or TV screen for too long, because it strains the eyes.
- Make sure there is enough light when you read, write or watch TV.
4. How do optical and non-optical aids help visually challenged persons?
Optical aids include special lenses and telescopic aids. Lenses help correct weak vision, and telescopic aids help students see the blackboard and classroom demonstrations from a distance.
Non-optical aids do not depend on the eye lens. Tactual aids, like Braille, use the sense of touch, so a person can read and write using raised dots. Auditory aids use the sense of hearing, and they include audio cassettes, audio books and CDs. Some computers and devices also have software that turns written text into speech. Together, these aids help visually challenged persons study, work and become independent.
5. What is the Braille system? How does it help visually challenged persons?
Braille is a system of reading and writing by touch, using patterns of raised dots on paper. It was created by Louis Braille. Each letter or symbol is made from a “cell” of up to six dots, arranged in two columns of three, and these dots can be combined in 63 different patterns. The patterns stand for letters, numbers, punctuation marks and even maths and science symbols, and Braille exists for many languages. A visually challenged person reads by running their fingertips over the dots, which helps them read, write and learn on their own.
Mistakes to avoid in the exam
Students often write that the image on the retina is erect. It is real, inverted and diminished, and the brain flips it.
Another common slip is mixing up myopia and hypermetropia. Remember: in myopia, the image forms in front of the retina and a concave lens helps. In hypermetropia, the image forms behind the retina and a convex lens helps.
Do not say that the image distance changes when the object moves. It stays fixed. It is the focal length of the lens that changes.
Finally, in the eye diagram, label every part clearly and draw the lens and retina properly, because these are the parts that carry marks.
Frequently asked questions
Why do we have two kinds of cells in the retina?
Rod cells help us see in dim light, and cone cells help us see colours in bright light. Together they let us see well in different conditions.
What causes cataract?
The eye lens gradually turns cloudy, so light cannot pass through it properly and vision becomes blurred. It is usually treated with a simple operation.
Why does the pupil change size?
The iris makes the pupil smaller in bright light to protect the eye, and larger in dim light to let more light in.
How can we protect our eyes while studying?
Sit in good light, keep the book at a comfortable distance, take short breaks, and avoid reading in moving vehicles or in the dark.
Written for students following the DAV Science book for Class 8. Please cross-check answers with your own textbook and teacher.