Question

Without his glasses, Isaac can see objects clearly only if they are less than 4.7 m from his eyes.

Answer 1

Answer:

The focal length is  $f = -4.721 \ m$

Explanation:

From the question we are told that

  The  image distance  is  $v = 4.7 + (2.1 *10^{-2}) = - 4.721 \ m$ the negative means  image is formed behind the lens

Generally the object distance is  $u = \infty$ this mean the light ray coming from the object are parallel to each other

So from lens equation

       $\frac{1}{\infty } - \frac{1}{4.721} = \frac{1}{f}$

=>   $f = -4.721 \ m$