Ray Optics and Optical Instruments Part 3

Two light beams fall on a transparent material block at point 1 and 2 with angle θ1 and θ2, 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,‌‌d=4√3‌cm and θ1=θ2=cos−1(‌

n2

2n1

), where refractive index of the block n2> refractive index of the outside medium n1, then the thickness of the block is ______ cm .