I did a demo using some plastic piece and the laser. At certain angles, the laser beam goes right through the block, but as I increase the angle, at a certain point, the laser stops going through the block and reflects off the inside of the block! This is called total internal reflection. It can happen whenever light moves from a shower medium into a faster medium. The angle when this happens is called the critical angle.
Here are some notes:
Handout
And some examples:
This is a photo from under a swimming pool.
At large angles (bottom of photo), the water reflects the bottom of the pool (total internal reflection). At small angles (top of the photo), the water lets light from above the pool get through. The critical angle is the line where the colour changes from black to blue.
High speed internet requires fibre optic cables, which use total internal reflection along plastic wires to carry information at high speeds. See how the light only comes out the end of the plastic wires? Endoscopes also use the same idea to let you see inside the human body using a plastic wire.
Here's what laser beams would look like as they bounces around inside a plastic fibre optic wire.
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