Two light beams fall on a transparent material block at point 1 and...

Two light beams fall on a transparent material block at point 1 and 2 with angle (\theta_1) and (\theta_{2^{\prime}}) respectively, as shown in figure. After refraction, the beams intersect at point 3 which is exactly on the interface at other end of the block. Given : the distance between 1 and 2, ($\mathrm{d}=$4 $\sqrt{3} \mathrm{~cm}$) and (\theta_1=\theta_2=\cos ^{-1}\left(\frac{\mathrm{n}_2}{2 \mathrm{n}_1}\right)$), where refractive index of the block ($\mathrm{n}}_{2}\gt) refractive index of the outside medium (\mathrm{n}}_{1}), then the thickness of the block is ________ cm.