Snells Law prism - from theta2 -> theta3? Hi, I was wondering if there's a formula to go straight from theta1 to theta 4, when the apex angle is known. Thanks [PLAIN]http://cord.org/cm/leot/course06_mod07/Fig3.gif [Broken] My problem is getting from theta2 to theta3. Thanks ps. This is not a homework question, i didnt even take physics in school when i had the chance!