Question

If a nearsighted person has a far point df that is 3.50m from the eye, what is the focal length f1 of the contact lenses that the person would need to see an object at infinity clearly?

Answer 1

Answer:

$-3.50$ meter

Explanation:

A nearsighted person uses concave lens.

Given -

Far point i.e distance of object $= - 3.5$ meter

Distance of image is not given . Hence we will assume that the image can be formed at any distance. We will take "di" $=$ infinity

As we know

$\frac{1}{f} = \frac{1}{di} + \frac{1}{df}$

Substituting the given values in above equation, we get -

$\frac{1}{f} = \frac{1}{infinity} + \frac{1}{-3.50}\\ \frac{1}{f} = 0 + \frac{1}{-3.50}\\ f = -3.50$

Hence, the focal length of the contact lens is equal to $-3.50$ meter