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## Physics Tutorials

### Optic Fiber and Refraction

#### Refraction

Refraction is the turning or bending of any wave, such as light or sound, when it passes from one medium to another of different optical density.

Light travels with different speeds in different substances (media). When light passes from a medium where it travels slowly to another medium where it travels quickly it bends or refracts away from the normal (an imaginary line perpendicular to the surface). The angle of light with respect to the normal line in the first medium is called the incident angle, in the second medium is called the refracted angle. As the incident angle becomes greater the refracted light also bends farther away from the normal line. At one particular angle, called the critical angle, the refracted light will not pass into the second medium but will travel along the boundary between the two media. If the angle of incidence is greater than the critical angle complete reflection will happen - the light is trapped in the first medium.

The index of refraction is a measure of how slowly light travels in a medium. The index of refraction compares the speed of light in a vacuum to the speed of light in a medium. The higher the index of refraction for a medium, the more slowly light travels in that substance. The greater the difference in index of refraction for two media, the more refraction occurs between them.

#### Optic Fiber

Optic fibers are long, thin strands of very pure glass, or plastic about the diameter of a human hair. They are used to transmit light. They are actually made of several layers. In an optic fiber the light will travel through the core (high index of refraction). Light is trapped in the core by a coating on the core called cladding. Cladding has a lower index of refraction. Light constantly reflects because the light is always at an angle greater than the critical angle. Light reflects from the cladding even if the optic fiber is bent in a complete circle.

Optic fibers can be used for long distances and is less expensive to use than copper wires. They are thin and flexible, and great for medical surgeries because you don't have to peel all the skin back to see torn muscles, tendons, cartilage, etc.

#### Question

Would light travel more quickly in the material making up the core of an optic fibre, or in the material making up the cladding?

Answer: Light would travel more quickly in the cladding as evidenced by its lower index of refraction.

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