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Refraction of light: Total internal reflection

Refraction of light: Total internal reflection

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If rays of light always travel in a straight line, how can we see things that do not emit light?

Refraction of light: Total internal reflection

Exactly how much the ray of light is refracted depends on the ratio between the speed of light in the two materials. Here's a table, showing how much more time light needs to travel a certain distance through different materials compared to vacuum. It takes one and half times as much time for light to go through glass than through vacuum, and more than twice as long to go through diamond. We call the numbers in this table the refractive indices. Exactly how much the ray of light is refracted depends on the ratio between the speeds of light in the two materials.

You measure the refractive index like this: Draw a line perpendicular to the surface hit by the ray of light. We call this line the normal. Now, you can measure the angle between the normal and the ray hitting the glass -- the angle of incidence. Then measure the angle between the normal and the refracted ray -- the angle of refraction. A large difference between those two angles means a strong refraction.

If the ray of light enters an optically denser material -- one with a higher refractive index -- the angle of refraction is smaller than the angle of incidence. But if a ray of light enters a material with a lower refractive index, the angle of refraction is greater than the angle of incidence. You can see how this works even without a table of refractive indices. Just take a look at a straw in a glass of water -- it looks like it's broken. But that's just the rays of light refracting.

Because water, just like glass, has a higher refractive index than air. This ray of light is traveling from glass to air. Right now it's perpendicular to the surface and doesn't bend at all. It travels straight. But now we turn it a little bit.

so the angle of refraction is greater. Air has a lower refractive index than glass, If we increase the angle of incidence a little more. What happens now? If the angle of incidence increases enough we reach the critical angle. Now the light ray bends along the surface of the glass.

Exactly what angle is critical depends on the materials from and to which the light is traveling. From glass to air, the angle is 41 degrees. What do you think will happen if we increase the angle of the ray of light past the critical angle? Now the light isn't bent anymore. It's reflected!

This is called total internal reflection. When the light hits the surface at a large enough angle to the normal, and it is traveling from a material with a higher optical density to another material with a lower optical density, the rays of light bounce back into the optically denser material! This is useful. In a fiber optic cable information is sent in the form of rays of light. It could be a phone call or an internet connection.

The fibers are made of glass or plastic and are just a bit thicker than a strand of hair. Total internal reflection makes the light bounce between the cable's walls, even if the cable turns. We can send more information over longer distances through a fiber optic cable than it is possible with ordinary copper wires. This works just like a fiber optic cable! The rays of light bounce off the inside of the stream of water.

It's like they don't want to come back to the air, so they go back into the water instead. Total internal reflection! Total internal reflection!