Question

near point of a person is 50 cm. in order to get normal vision what kind of spectacles should to use

A. convex lens and focal length is 50cm

B. convex lens and focal length is 25cm

C. concave lens of a focal length is 50cm

D. concave lens of a focal length is 25cm​

Answer 1

Answer:

concave lens of focal length 50 cm...

The near point of an elderly person =50cm from the eye.For minimum to see object placed at normal point, its image should be formed 50 cm from the eye....

Answer 2

Given:

Near point of a person is 50 cm.

To find:

The type of spectacles she should use in order to get a normal vision.

Calculation:

We know that for a normal person , the near point from the eye is 25 cm. But in this woman , the near point has shifted away to 50 cm.

So, applying Lens formula:

$\therefore \: \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$= > \: \dfrac{1}{f} = \dfrac{1}{( - 50)} - \dfrac{1}{( - 25)}$

$= > \: \dfrac{1}{f} = - \dfrac{1}{50} + \dfrac{1}{25}$

$= > \: \dfrac{1}{f} = \dfrac{ 2 - 1}{50}$

$= > \: \dfrac{1}{f} = \dfrac{1}{50}$

$= > f = 50 \: cm$

So, the lady has to use a convex lens of 50 cm focal length in order to get normal vision.