This applet is a sort of tutorial which explains the reflection and the refraction of waves by the principle of Huygens. Explanations of each of the steps are provided in the text box. Whenever a step is finished, press the "Next step" button! You can stop and continue the simulation by using the "Pause / Resume" button. The three text fields make it possible to vary the index of refraction of both media and the angle of incidence. The medium with the smaller index of refraction (the bigger phase velocity) is painted yellow, the other blue.