Chapter 11: Human Eye and Colourful World

Internal Question and Answer

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

Answer: The power of accommodation of the eye is the ability to adjust the focus and maintain clear vision when viewing objects at different distances.

2. 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?

Answer: A myopic eye cannot see distant objects clearly. It is corrected using a concave lens (diverging lens) of suitable power. The concave lens forms a virtual image of distant objects at the far point of the eye (1.2 m), enabling clear vision.

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

Answer: For a human eye with normal vision, the near point is about 25 cm from the eye, which is the minimum distance at which objects can be seen clearly. The far point is at infinity, meaning very distant objects can be seen clearly without strain.

4. 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?

Answer: The child is likely suffering from Myopia (short-sightedness), in which distant objects appear blurred. It can be corrected by using a concave lens of suitable power, which helps the eye focus distant images clearly on the retina.

10.6.2:  Why is the colour of the clear Sky Blue?

Answer: The colour of the clear sky is blue due to scattering of light. Air molecules scatter shorter wavelengths of light (blue) more than longer wavelengths (red). As a result, blue light is scattered in all directions and reaches our eyes, making the sky appear blue.

Exercise Questions and Answers

1. 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.

Answer:  (b) Accommodation

[ The human eye changes the focal length of its lens to focus objects at different distances. This ability is called Accommodation.]

2. The human eye forms the image of an object at its

(a) cornea.    (b) iris.      (c) pupil.      (d) retina.

Answer:   (d) retina

[ The image of an object is formed on the retina of the human eye, where light-sensitive cells detect the image and send signals to the brain.]

3. 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.

Answer:  (c) 25 cm

[ The least distance of distinct vision for a normal young adult is about 25 cm, which is the closest distance at which objects can be seen clearly without strain.]

4. 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.

Answer :  (c) ciliary muscles

[ The change in focal length of the eye lens is controlled by the ciliary muscles, which adjust the shape of the lens to focus on objects at different distances.]

5. 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?

Answer: (i) For distant vision correction : Here, and  

We have, 

 

The focal length for correcting distant vision is approximately  – 0.18 meters .

(ii) For near vision correction : Here,  and

We have, 

 

The focal length for correcting near vision is + 0.67 meters.

6. 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?

Answer: To correct myopic (near-sightedness), a concave lens is used .

Here, 

We have, 

 

The nature and power of the lens required to correct the problem are a diverging (concave) lens with a power of approximately  – 1.25 D .

7. 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.

Answer: The ray diagram for correction of a hypermetropic eye:

    

To correct hypermetropia (farsightedness), a converging lens (convex lens) is used.

The near point of a hypermetropic eye is the closest distance at which the person can see objects clearly. In this case, the near point is given as 1 meter.

The near point of a normal eye is 25 cm

Here,  ,

Using lens formula ,  

 

 

Since the focal length is negative, it indicates a converging lens (convex lens).

We have, 

 

 

The power of the lens required to correct the hypermetropic eye is + 3D .

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

Answer:  A normal eye cannot see objects closer than 25 cm clearly because the eye lens cannot become more convex beyond a limit. At distances less than 25 cm, the image cannot be focused on the retina, so it appears blurred.

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

Answers: When we increase the distance of an object from the eye, the image distance in the eye increases as well. This means that the image formed on the retina moves farther away from the eye's lens.

10. Why do stars twinkle?

Answers: Stars twinkle due to atmospheric refraction of starlight. As light from a star passes through different layers of Earth’s atmosphere, it bends continuously because the refractive index keeps changing. This causes the apparent position and brightness of the star to fluctuate, making it appear to twinkle.

11. Explain why the planets do not twinkle.

Answers: Planets do not twinkle because they are closer to Earth and appear as extended sources of light, not as point sources. The light coming from different parts of the planet undergoes fluctuations due to atmospheric refraction, but these fluctuations cancel out each other. Hence, the total brightness remains constant, and planets do not twinkle.

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

Answers: The sky appears dark to an astronaut because there is no atmosphere in space to scatter sunlight. On Earth, the blue color of the sky is caused by Rayleigh scattering of sunlight by air molecules. In the absence of an atmosphere, no scattering occurs, so the sky looks black.