This is a way that you can do this experiment without using a protractor. Consider a ray of light passing through the two semicircles shown. The left hand circle has the same refractive index as air (1.0) and the right has refractive index the same as glass (1.5). The ray of light is arranged so that it passes though the centre of the circle. This is all a bit contrived but you can make it easily with the simulation...