The Human Eye

 

Science - Class 8

Chapter 11 : The Human Eye

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Part A Part B Part C Part D Part E

A. Fill in the blanks.

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

ANS: iris

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

ANS: retina

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

ANS: colours

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

ANS: hypermetropia

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

ANS: concave, convex

B. Write True or False for the following statements.

1. Iris is the muscular assembly that controls the size of the pupil.

ANS: True

2. The eye lens forms a virtual, erect and diminished image of an object, on the retina.

ANS: 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.

ANS: True

4. Cinematography makes use of the ‘persistence of vision’ of the human eye.

ANS: True

5. For a normal human eye, the range of vision is from (nearly) 25 cm to infinity.

ANS: True

6. Myopia is caused due to excessive curvature of cornea.

ANS: 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—

•            pupil

•            iris

•            retina

•            ciliary muscles

ANS: ciliary muscles

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

•            1/8 second

•            1/24 second

•            1/16 second

•            1/32 second

ANS: 1/16 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—

•            persistence of vision

•            far sightedness

•            near sightedness

•            power of accommodation

ANS: power of accommodation

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

•            a book held in hands.

•            a text message on the mobile phone.

•            a street signboard across the street.

•            an e-mail, on the computer screen.

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—

•            near sightedness

•            far sightedness

•            cataract

•            nightblindness

ANS: cataract

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

•            visual aid

•            auditory aid

•            tactual aid

•            electronic aid

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: The cornea acts as a protective outer layer for the human eye.

2. The pupil appears black. Why?

ANS: The pupil appears black because no light is reflected from it.

3. Differentiate between:

(a) rod-shaped and cone-shaped cells

ANS:

Rod-shaped cells: These are light sensitive cells present in the retina that respond to the amount of light. They help in vision in dim light.

Cone-shaped cells: These are light sensitive cells present in the retina that respond to colours. They help in colour vision and function mainly in bright light.

(b) near point and far point of human eye

ANS:

Near point: The nearest point up to which the eye can see objects clearly without strain. For a normal eye, it is about 25 cm.

Far point: The farthest point up to which the eye can see objects clearly. For a normal eye, it is at infinity.

4. State the difference between:

(a) myopia and hypermetropia

ANS:

Myopia: It is the defect of vision in which a person cannot see distant objects clearly, but can see nearby objects clearly. It is corrected by a concave lens.

Hypermetropia: It is the defect of vision in which a person cannot see nearby objects clearly, but can see distant objects clearly. It is corrected by a convex lens.

(b) tactual aids and auditory aids

ANS:

Tactual aids: These aids help visually challenged persons by using the sense of touch, e.g., Braille.

Auditory aids: These aids help visually challenged persons by using the sense of hearing, e.g., audio cassettes, tape recorders and audio books.

5. Define the terms:

(a) Blind spot on the retina

ANS: The point on the retina where the optic nerve joins it and where no rods and cones are present is called the blind spot. It is insensitive to light.

(b) Persistence of vision of human eye

ANS:The ability of the human eye to continue seeing the image of an object for about 1/16 second even after the object is removed is called persistence of vision.

6. Give the meaning of the terms:

(a) Power of accommodation of the human eye

ANS: The ability of the eye lens to adjust its focal length so that objects at different distances can be seen clearly is called the power of accommodation.

(b) Least distance of distinct vision

ANS: The minimum distance at which an object can be seen clearly without strain is called the least distance of distinct vision.

(c) Range of vision

ANS: The distance between the near point and the far point of the eye is called the range of vision.

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

ANS: The image distance in the eye remains nearly constant because the image is always formed on the retina. The eye adjusts the focal length of its lens to focus the image on the retina.

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 is formed beyond the retina.

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

ANS: Sahil’s least distance of distinct vision is more than 25 cm.

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 person’s eyeball is likely elongated (longer than normal). A concave lens is needed to correct his vision.

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

ANS: Cornea is the transparent curved outer layer of the eye. It protects the eye and allows light to enter.

(b) iris

ANS: Iris is the dark coloured muscular diaphragm behind the cornea. It controls the size of the pupil and regulates the amount of light entering the eye.

(c) pupil

ANS: Pupil is the small opening in the centre of the iris through which light enters the eye.

(d) eye lens

ANS: Eye lens is a transparent convex lens that focuses light on the retina.

(e) ciliary muscles

ANS: Ciliary muscles hold the eye lens and change its curvature to adjust its focal length.

(f) retina

ANS: Retina is the light sensitive screen of the eye on which the image is formed. It contains rods and cones.

(g) optic nerves

ANS: Optic nerve carries visual signals from the retina to the brain.

2. Explain how we perceive objects as they are.

ANS: Light reflected from an object enters the eye through the cornea and pupil and falls on the eye lens. The eye lens forms a real, inverted and diminished image on the retina. The rods and cones of the retina convert this image into electrical signals. These signals are sent to the brain through the optic nerve. The brain interprets these signals and makes us perceive the object in its proper shape, size and position.

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

ANS:

1.         Have regular eye check-ups and use proper spectacles if advised.

2.         Never look directly at the Sun or any powerful light source.

3.         Read and write only in proper light.

4.         Avoid watching screens continuously for too long.

5.         Eat a balanced diet rich in fruits and vegetables, especially vitamin A.

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

ANS:

Visually challenged persons are helped by both non-optical and optical aids.

Non-optical aids include tactual aids such as Braille, which help them read and write by touch, and auditory aids such as audio books, tape recorders and screen-reading software, which help them learn by hearing.

Optical aids include suitable lenses and telescopic aids, which help partially visually challenged persons improve vision for reading, seeing demonstrations and viewing distant objects. These aids help them develop their abilities and become more independent.

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

ANS:

Braille is a system of writing for visually challenged persons developed by Louis Braille. It is based on patterns of raised dots arranged in cells. Each character is represented by a combination of raised dots that can be felt by the fingertips.

Braille helps visually challenged persons to read and write using the sense of touch. It enables them to learn languages, mathematics and other subjects independently.

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