Question

least distance of distinct vision of a long sighted man is 40cm he wishes to reduce it to 25 CM of lens then what is focal length of lens​

Answer 1

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Answer 2

Answer:

The convex lens of focal length 50 cm.

Explanation:

Step 1: To solve this question, we need to use the lens formula. On substituting the values of the image and the object distances given in the question, we will get the focal length of the lens from which the power can be calculated. The sign of the focal length will tell the type of the lens used.

Step 2: Formula used: The formulae used to solve this question are given by

1.

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$,

here f is the focal length of a lens, v is the image distance, and u is the object distance.

2. $P=\frac{1}{f}$,

Step 3: here P is the power of a lens, and f is its focal length.

Given, $\mathrm{u}=\infty$

Image distance,$\mathrm{v}=50 \mathrm{~cm}$

From lens formula,

$\frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}}$

$\frac{1}{50}-\frac{1}{\infty}=\frac{1}{f}$

$\mathrm{f}=+50 \mathrm{~cm}$

Hence, the convex lens of focal length 50 cm.

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