Thursday 8-1-96
The relevant section in the book is 21.4
Homework from this session : Chapter 21 - 39, 45, 59, 64
When light travels from a low refractive index material to a higher refractive index material, some light will be reflected, but some will always be transmitted into the second material, no matter what the angle of incidence. Going the other way is a different story; going from high n to low n, there will be some angle of incidence (known as the critical angle) beyond which no light will be transmitted, so all the light will be reflected. This is known as total internal reflection.
The critical angle is the theta1 at which theta2 works out to be 90 degrees. Any theta1 less than that (as long as n1 > n2), theta2 will be less than 90, so light will be happy to be transmitted to the second material. For a theta1 greater than the critical angle, Snell's law will give an impossible result for theta2...Snell's law applies to transmitted light, so no light is transmitted in that case. The critical angle, thetaC can be calculated by putting theta2 equal to 90 degrees in the Snell's law equation. This gives:
sin thetaC = n2/n1 (n1>n2)
Optical fibers are based entirely on this principle of total internal reflection. An optical fiber is a flexible strand of glass. A fiber optic cable is usually made up of many strands, each carrying its own signal...the signal is usually bursts of laser light, information being carried in the pattern of the bursts. The laser light travels along the optical fiber, reflecting off the walls of the fiber. With a straight, or smoothly bending, fiber, the light will hit the wall at an angle higher than the critical angle, so it will all be reflected back into the fiber. Light can undergo many reflections before reaching the far end of the fiber, but the loss of light is relatively small because on each reflection it will be totally reflected, rather than transmitted out of the fiber.