Answer
$\dfrac{f}{2}$
Work Step by Step
As per the given problem, we have:
$f=k \dfrac{1}{L}$
Here, $k$ is defined as constant of proportionality
when the length gets doubled then we have
$f_n=\dfrac{k}{2L}$
Plug the given data in the above expression, we get
$f_n=\dfrac{k}{L_n}=\dfrac{k}{2L}$
This gives: $f_n=\dfrac{f}{2}$
