A student with a normal eye observes the reading on a vernier scale using a magnifying glass of focal length 10 cm. What are the minimum and the maximum distances between the scale and the magnifying glass at which he can read the scale when viewing through the magnifying glass?

$\begin{array}{l} \text { Focal length }=+10 \mathrm{~cm} \\ \text { Since it is magnifying lens it is convex len: } \\ \text { The image distance vary from least } \\ \text { distance of distinct vision to infinity. } \\ \begin{array}{l} \dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u} \\ \text { so object distance is calculated as } \\ \dfrac{1}{10}=\dfrac{1}{-25}-\dfrac{1}{u} \\ \dfrac{1}{4}=\dfrac{1}{25}-\dfrac{1}{10} \\ \dfrac{1}{u}=\dfrac{-2-5}{50} \end{array} \end{array}$

$\begin{aligned}u &=-\dfrac{50}{7} \\u &=-7.14\mathrm{~cm}\end{aligned}$

At infinity

$\begin{aligned}\dfrac{1}{10}=\dfrac{1}{\infty} &\dfrac{1}{4} \\u &=-10 \mathrm{~cm}\end{aligned}$

So B) $7.14 \mathrm{~cm}, 10 \mathrm{~cm}$ is the

option