The equi-convex lens has a focal length ‘ $f$ ’. If the lens is cut along the line perpendicular to principal axis and passing through the pole, what will be the focal length of any half part?

• A

  $\frac{f}{2}$
• Correct answer
  B

  $2f$
• C

  $\frac{3f}{2}$
• D

  $f$

Solution

$\frac{1}{f}=\left(n-1\right)\left(\frac{2}{R}\right)$

$\Rightarrow \frac{1}{f\text{'}}=\left(n-1\right)\left(\frac{1}{R}\right)$
$\therefore f^{\text{'}}=2f$

Hence, the correct answer is option: 'B'.