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Chapter Review:

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• The human eye is one of the most valuable and sensitive sense organs.
• An image on a light-sensitive screen called the retina. Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball.
• The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
• Iris is a dark muscular diaphragm that controls the size of the pupil.
• The pupil regulates and controls the amount of light entering the eye. The eye lens forms an inverted real image of the object on the retina.
• The ability of the eye lens to adjust its focal length is called
  accommodation.
• The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye.
• The farthest point upto which the eye can see objects clearly is
  called the far point of the eye.
• The near point is about 25 cm.
• The farthest point is infinity for a normal eye.
• The crystalline lens of people at old age becomes milky and
  cloudy. This condition is called cataract.
• A human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes.
• There are mainly three common refractive defects of vision. These
  are (i) myopia or near-sightedness, (ii) Hypermetropia or farsightedness, and (iii) Presbyopia.
• A person with myopia can see nearby objects clearly but cannot see distant objects  distinctly .
• A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
• The power of accommodation of the eye usually decreases with ageing.The near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.
• Eyes must be removed within 4-6 hours after death.
• The angle between its two lateral faces is called the angle of the prism.
• The emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation.
• The splitting of light into its component colours is called dispersion.
• The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.
• The red light has a wavelength about 1.8 times greater than blue light.
• Scattering of light causes the blue colour of sky and the reddening of the Sun at sunrise and sunset.

 

Text-book Questions-Answer:

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Page 190 NCERT Book.

Q1. What is meant by power of accommodation of the eye?

Ans: Humans have to organize their eyes to see distant and near objects completely. In this way, the ability of the human eye lens to adjust its focal length angle is called adjusting capacity.

Q2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

Ans: Concave lens.

Q3. What is the far point and near point of the human eye with normal vision?

Ans: For normal vision, the far point is at an infinite distance from the eye and the near point is at a distance of 25CM from the eye.
Q4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

Ans: This student has myopia. Myopia is corrected by a concave lens of suitable power.

Exercise : Chapter 11

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Q1. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
(a) presbyopia.
(b) accommodation.
(c) near-sightedness.
(d) far-sightedness.

Ans: (b) accommodation.

Q2. The human eye forms the image of an object at its
(a) cornea. (b) iris (c) pupil. (d) retina.

Ans: (d) retina

Q3. The least distance of distinct vision for a young adult with normal vision is about
(a) 25 m.

(b) 2.5 cm.

(c) 25 cm.

(d) 2.5 m.

Ans: (c) 25 cm

Q4. The change in focal length of an eye lens is caused by the action of the
(a) pupil.

(b) retina.

(c) ciliary muscles.

(d) iris.

Ans: (c) ciliary muscles

Q5. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Ans: 

Q6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

Ans: 

Q7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

Ans: 

Q8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Ans: The minimum distance of distinct vision of a human is 25cm. Rays of light reflected off an object placed at a distance of less than 25 cm will not make the object visible on the retina. Because the capacity of the human eye cannot be increased by 25 cm.

Q9. What happens to the image distance in the eye when we increase the distance of an object from the eye?

Ans: Image distance always remains the same. The reason for this is that the distance of the object, the focal length of the lens of the human eye is adjusted in such a way that the image is formed on the retina.

Q10. Why do stars twinkle?

Ans: The refractive index of the Earth's atmosphere is constantly changing. The light of the stars entering the eyes remains irregular due to constant refraction and due to that flicker the stars appear to twinkle.

Q11. Explain why the planets do not twinkle.

Ans: Earth's distance from the planets is very less. Planets are stores of light. The light rays that come from the planets do not have refraction. Their position does not change with the proximity and the store of light, so they do not appear to twinkle.

Q12. Why does the Sun appear reddish early in the morning?

Ans: The sun is on the horizon at sunrise or sunset. In that case, the sun's rays first reach the thick layers of air in the earth's atmosphere, after that to our eyes. Most of the light of short wavelength is scattered by the particles of the atmosphere. In this way only long light rays (red) can enter our eyes and we see the sun as blood-stained.

Q13. Why does the sky appear dark instead of blue to an astronaut?

Ans: Due to the absence of an atmosphere in space, there is no scattering of light, because only the fine particles of the air scatter the light. This is the reason why the sky appears black to the astronauts.

Chapter-11. Human Eye and Colourful World

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Que. – What is iris? Write its function.

Ans. – The colour part of eye is known as iris . it gives blue black & brown coulor to the eyes. It has a hole in the middle named pupil which regulates the amount of light entering in to the eye.

Que. – Write the function of cornea.

Ans. – It refracts most of the light rays into the eye.

Que. – Write the name of light sensitive part of the human eye.

Ans. – Retina.

Que. – Write the function of optic nerve.

Ans. – Its function is to carry electrical signals to the brain.

Que. – Name the eye muscles which have ability to change the focal length and curvature of lens.

Ans. – Ciliary muscles.

Que – What do you understand by power of accommodation?

Ans. – The ability of the eye lens to adjust its focal length is called power of accommodation.

Que. - The wavelength of which colour is the longest.

Ans. – Red.

Que. - Write the name of light sensitive part of eye.

Ans. – Retina.

Que. - What is called the band of the colored components of a light beam?

Ans. – Spectrum.

Que. - Write two phenomenons caused by scattering of light.

Ans. –

(I)   looking sky blackish to an astronaut.

(ii)    looking the sun reddish during the sun rising and setting. 

Que. - What do you mean by power of accommodation? What happens, The eye loses its power of accommodation?

Ans. - The ability of the eye lens to adjust its focal length is called power of accommodation. When the eye loses its power of accommodation, the eye does not be able to adjust it focal length resulting in not to see the objects of various focal length.

Que. – What do you understand by scattering of light. Name two phenomena caused by scattering of light.

Ans.  - The Earth’s atmosphere has many types of colloidal particles, which makes the path of a beam of light visible. Such a phenomenon is known as scattering of light.

Two phenomenons are as follow.

(I)    looking sky blackish to an astronaut.

(ii)    looking the sun reddish during the sun rising and setting.

Que: – Why does sky look dark to an astronaut?

Ans. – The space does not have air or other particles, there is no atmosphere like the earth. There is only vacuum. These particles are responsible for scattering of light . So there scattering of light does not take place and sky looks dark.

Que. -  Why do the small particles of air scatter blue colour more speedily than red colour? 

Ans. – The small particles of air scatters more effectively blue colour and red colour. As these particles are more effective scattering light of shorter wavelength of blue colour than longer wavelength of red colour.   

Que. – What is presbyopia ? give region for this defect? How does it remove?

Ans. – When the eyes have lost completely their power of accommodation. The near point gradually recedes away. The persons find it difficult to see near by objects comfortably and distinctly without corrective eye-glasses. This defect is called presbyopia.  

Que. -  A person can’t read a news paper clearly but he can see comfortably the newspaper placed at distance of 80 cm. 

(i) By which defect of vision does this person suffer?

Ans. – He suffers with hypermetropia.

(ii) What is correction of this defect of vision?

Ans – By using convex lens of appropriate power.

(iii) Give two regions for this defect of vision?

Ans –

(a)    the focal length of the eye lens is too long.

(b)   The eyeball has become too small.

(iv) Show to draw the light ray graph of this defect of vision, How is this corrected?

A – 

Long Question with answer: 

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Q1. A child while playing with his father's spectacles burnt a hole in a piece of paper by focusing a small image of sun on it. 

(i) What defect of vision his father is suffering from? 

(ii) Write two cause for this defect? 

(iii) Draw a ray diagram to show image formation by the defective eye?

(iv) Draw a ray diagram showing corrected eye using proper lens. 

Ans: 

(i) Hypermetropia

(ii) (a) the focal length of the eye lens is too long.

   (b) the eyeball has become too small.

(iii) Ray diagram to show image formation by the defective eye.

(iii) This defect can be corrolated by convex lens.