Two light beams fall on a transparent material block at point 1 and 2 with angle and , respectively, as shown in the figure. After refraction, the beams intersect at point 3 which is exactly on the interface at the other end of the block. Given: the distance between 1 and 2, cm and , where is the refractive index of the block and is the refractive index of the outside medium, then the thickness of the block is cm.