Question

In an experiment, students need to get an upright image of an object using a lens of focal length 25 cm. The experiment was set up by putting the lens and the object at a distance of 10 cm. Students of group A used a convex lens and group B used a concave lens for their experiment. The ratio of magnification obtained by group A and group B is:

• A. $\frac{3}{7}$
• B. $\frac{7}{3}$
• C. 3
• D. 1

Answer 1

The correct option is B $\frac{7}{3}$

Using sign convention,
$u=-10cm$,$f_A=+25cm\left(convexlens\right)$, $f_B=-25cm\left(concavelens\right)$

Using lens formula,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

For group A,
$\frac{1}{25}=\frac{1}{v_A}-\frac{1}{-10}$

$v_A=-50/3cm$

For group B,
$\frac{1}{-25}=\frac{1}{v_B}-\frac{1}{-10}$

$v_B=-50/7cm$

Magnification, $m=\frac{v}{u}=\frac{Heightofimage\left(I\right)}{Heightofobject\left(O\right)}$

Therefore, $m_A=\frac{v_A}{u_A}=\frac{(-50/3)}{(-10)}=5/3$

Similarly, $m_B=\frac{v_B}{u_B}=\frac{(-50/7)}{(-10)}=5/7$

Hence,
$\frac{m_A}{m_B}=\frac{(5/3)}{(5/7)}=7/3$