Answer
The new length becomes quadruple the first length.
Work Step by Step
As per the given problem, we have:
$P=k\sqrt l$
Here, $k$ is defined as constant of proportionality
when the period gets doubled then we have
$P_2=2 P_1$
Plug the given data in the above expression, we get
$k\sqrt l_2=2(k\sqrt l_1)$
This gives: $l_2=4l_1$
Hence, the new length becomes quadruple the first length.
