Question

A person needs a lens of power
-5.0 dioptres for correcting his
distant vision. What is the focal
length of the lens required for
correcting distant vision?
​

Answer 1

$\boxed{\green{Focal\:Length={\frac{1}{Power}}}}}$

GIVEN:-

Power = -0.5 D

_______________________________

Using this formula, we get

$f = \frac{1}{ - 5}$

$f = - 0.2m$

Hence,

f = - 20 cm

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Answer 2

The focal  length of the lens required for  correcting distant vision is 20 cm and it is a concave lens.

Explanation:

Given that,

Power of the lens, P = -5 D

To find,

Focal length of the lens.

Solution,

The power of a lens in terms of its focal length is given by :

$P=\dfrac{1}{f}$

$f=\dfrac{1}{P}$

$f=\dfrac{1}{-5}$

f = -0.2 m

or

f = -20 cm

So, the focal  length of the lens required for  correcting distant vision is 20 cm and it is a concave lens. Hence, this is the required solution.

Learn more,

Power of a lens

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