A blade parallel faces is 5 mm thick. Taken to a microscope it appears that, to pass on the focus of a surface point to a point on the underside of the blade should move the cannon from the microscope 3mm. What is the refractive index of the material it is made the blade? 5/3 Thx EDIT: Sorry, i cant delete the post now, i'm nwe here e sorry again. I can't read the rules, but now read it.