Experiment with different incident angles and refractive indices. You can move the flashlight
around, and change the index of the top and bottom mediums. You can enter the index for other mediums.
Notice how the light bends toward the normal when the light enters a medium of greater refractive index, and away from the normal when
entering a medium of lesser refractive index.
Then notice what happens when you move the flashlight to a certain angle
in the medium with a higher refractive index. As you approach this critical angle, the refracted light is no longer transmitted across the medium boundary. For angles greater than the critical angle, all the light is reflected. This is called total internal reflection. For diamond into vacuum, it's about 24.5 degrees.
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