NCERT Class X Science Solutions: Chapter 11 – Human Eye and Colourful World Part 2

Doorsteptutor material for CBSE/Class-10 Hindi is prepared by world's top subject experts: fully solved questions with step-by-step explanation- practice your way to success.

Question 5:

A person needs a lens of power dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power diopter. What is the focal length of the lens required for correcting?

  1. distant vision,

  2. Near vision?

Answer:

For distant vision, for near vision The power P of a lens of focal length f is given by the relation (i) Power of the lens used for correcting distant vision Focal length of the required lens, The focal length of the lens for correcting distant vision is . (ii) Power of the lens used for correcting near vision Focal length of the required lens,

The focal length of the lens for correcting near vision is .

Question 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:

The person is suffering from an eye defect called myopia. In this defect, the image is formed in front of the retina. Hence, a concave lens is used to correct this defect of vision. Object distance, infinity Image distance, Focal length According to the lens formula, we know, A concave lens of power is required by the person to correct his defect.

Question 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:

A person suffering from Hypermetropia can see distinct objects clearly but faces difficulty in seeing nearby objects clearly. It happens because the eye lens focuses the incoming divergent rays beyond the retina. This defect of vision is corrected by using a convex lens. A convex lens of suitable power converges the incoming light in such a way that the image is formed on the retina, as shown in the following figure.

Result for Correction for hypermetropic eye

Correction for Hypermetropic Eye

Loading image

The convex lens actually creates a virtual image of a nearby object (N’ in the figure) at the near point of vision (N) of the person suffering from Hypermetropia.

The given person will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.

Object distance,

Image distance, Focal

Length, f

Using the lens formula,

A convex lens of power is required to correct the defect.

Question 9:

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

Answer:

Since the size of eyes cannot increase or decrease, the image distance remains constant? When we increase the distance of an object from the eye, the image distance in the eye does not change. The increase in the object distance is compensated by the change in the focal length of the eye lens. The focal length of the eyes changes in such a way that the image is always formed at the retina of the eye.