Question

The following graph depicts the inverse of magnification versus the distance between the object and lens data for a setup. The focal length of the lens used in the setup is

• A. 250 m
• B. 0.004 m
• C. 125 m
• D. 0.002 m

Answer 1

Answer: (B)

Magnification $( m )=+\frac{ v }{ u }$
From lens formula,
$\frac{1}{ v }-\frac{1}{ u }=\frac{1}{ f }$
$\Rightarrow \frac{ u }{ v }-1=\frac{ u }{ f }$
$\Rightarrow \frac{ u }{ v }=\frac{ u }{ f }+1$
$\therefore $ graph between $\left(\frac{ u }{ v }\right)$
[inverse of magnification] and u will be straight line with slope $\frac{1}{ f }$
From graph, slope $=250$
$\therefore f =\frac{1}{250} m =0.004\, m$