Also spring constant $(k) \propto \frac{1}{{{\rm{Length (}}L)}}$, when the spring is half in length, then $k$ becomes twice.
$\therefore T' = 2\pi \sqrt {\frac{m}{{2k}}} $
$\Rightarrow \frac{{T'}}{T} = \frac{1}{{\sqrt 2 }} $
$\Rightarrow T' = \frac{T}{{\sqrt 2 }}$


