Transparent objects do not form shadows. The light passes completely from the transparent objects thus these objects will not form shadow. ... In such objects, the light gets refracted thus, such objects forms shadow. The refraction is also the reason why we can see such objects.
i don't know if this helps but The refractive index of the material of the prism of an angle 60 degree is 1.62 for sodium light. there you go hope it helps
