Question

What focal length should the reading spectacles have for a person for whom the least distance of distinct vision is 50 cm ?

Answer 1

concave lens
concave lens

Answer 2

Answer:

The focal length of the reading lens should be 50.

Explanation:

The focal length for the reading spectacles with a vision distinction of 50 cm can thus be known as follows –

As given that for distinct vision i.e. distance from object u = 25  

Distinct vision as given i.e. distance of image v = 50

As we know that – $\bold{\frac{1}{f}=\frac{1}{v}-\frac{1}{u}}$

$\begin{array}{l}{\Rightarrow \frac{1}{f}=\frac{1}{v}-\frac{1}{u}} \\ {\Leftrightarrow \frac{1}{f}=\frac{1}{-50}-\frac{1}{-25}} \\ {\Rightarrow \frac{1}{f}=\frac{1}{-50}+\frac{1}{25}} \\ {\Rightarrow \frac{1}{f}=-\frac{1}{50}} \\ {\Rightarrow \bold{f=50}}\end{array}$