Total Internal Reflection 

This animation is very similar to the previous one. It allows you to explore Snell's law again, only now you can shine the laser beam from any direction you like. You are free to select the media for the upper and lower part of space from the drop-down lists. Now you should observe the following: shine the laser from the optically denser medium (which you can recognize by the darker blue color) into the lower n medium. Start at normal incidence, with the laser hitting the surface perpendicular, and move it to larger angles of incidence. Since the light is going from a higher n medium to a lower n medium, the beam is refracted *away* from the normal. Now you can observe that something strange happens when the refracted beam is at a right angle to the normal, i.e. going parallel to the interface between the two media. If you increase the angle further, there is no more refracted beam in the other medium, the beam is instead *reflected* from the interface, like from a mirror. This is called "total internal reflection". It is the basic principle that allows light to propagate in an optical glass fiber. Keep playing with the animation until you feel that you understand how total internal reflection works. Think about the following questions: What are the conditions for the two media for total internal reflection to occur? How could you calculate the smallest angle of incidence for which total internal reflection happens, as a function of the two indices of refraction n1 and n2? This angle is called the critical angle, and it is important for example for fiberoptics.      

 

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