This animation enables you to carry out a simulated experiment to measure the refractive index of a number of transparent materials using a determination of the critical angle. It also demonstrates total internal reflection.
Drag the ray box to change the angle of incidence at the surface of the material and then read the angle of refraction from the protractor scale.
Drag the slider to change the material. Four materials are available: water, glass, sapphire and diamond.
Notice that the protractor is graduated to allow you to read the angles of incidence and refraction directly.