Question

Suresh can see the objects clearly which are beyond 3m. So he consulted the doctor

and the doctor suggested him some lens.

a) What type of eye defect he has?

b) What kind of lens, doctor suggested him to overcome the eye defect?

c) What is the focal length of the lens?



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Answer 1

Answer:

a) Defect is farsightedness( hypermetropia)

b)Doctor suggested him to use the convex lens as the near point (25 cm) of his eye is shifted farther away

Answer 2

Answer:

The Correct Answers are listed as below.

a) He is suffering from Hypermetropia or farsightedness.

b) The doctor suggested him to wear convex lenses.

c) The focal length of convex lens used is 0.27 m.

Explanation:

Hypermetropia is the defect in which a person faces difficulty in focusing on objects which are nearby, but are clearly able to see the distant objects. The image in this case is formed behind the retina. Hence we use convex lens to converge the light rays entering the eyes, making it able to form o the retina.

For the focal length of image, we know that near point of a normal eye = 25 cm.

In this case, near point of the deceased eye = 3m = 300 cm

Hence, u = -25 cm and v = -300 cm    (by sign convention)

Therefore, from lens formula :

                                             1/f = 1/v - 1/u

1/f = (-1/300) - (-1/25)

1/f = 1/25 - 1/300

1/f = 1/27.2 cm

f = 27.2/100 m

F = 0.27 m.