$\Rightarrow {l_2} = {l_1}\left( {\frac{{{n_1}}}{{{n_2}}}} \right)$
$ = 50 \times \frac{{270}}{{1000}}$$ = 13.5\,cm$
${y_1} = {10^{ - 6}}\sin [100\,t + (x/50) + 0.5]m$
${y_2} = {10^{ - 6}}\cos \,[100\,t + (x/50)]m$
where $ x$ is expressed in metres and $t$ is expressed in seconds, is approximately .... $ rad$