Two Things:[br]1) If we can trace straight lines along the two paths of a ray, incident and transmitted, we can read the IOR off a straight line without angle measures.[br][br]But how to measure straight lines if the light is being refracted?[br]Consider a wedge of, say, glass. A short cylinder, like a block of good cheese. Cut it in half along a diameter.[br]Here is a clever shape! A ray passing through the flat face, and which strikes at the midpoint, will always exit[br]the other side in a direction perpendicular to the surface. It will experience no refraction, and the ray passed [br]outside the glass will have the direction of the transmitted ray.[br][br]And any beam striking the curved face which is aimed at the circle center will pass into the glass without refracting.[br]The ray leaving the flat face gives the refraction [i]from[/i] glass [i]to[/i] the outside medium. [br][br]2) To speak of the behavior of light as it passes through an object, we must consider [i]closed[/i] surfaces with known dimensions. Here is a gentle introduction (for me!).