Time taken by light to travel in two different materials $A$ and $B$ of refractive indices $\mu_A$ and $\mu_B$ of same thickness is $t_1$ and $t_2$ respectively. If $t_2-t_1=5 \times 10^{-10} s$ and the ratio of $\mu_A$ to $\mu_B$ is $1: 2$. Then the thickness of material, in meter is: (Given $v_A$ and $v_B$ are velocities of light in $A$ and $B$ materials respectively).