In the Discussion Questions you were told that optical fibers use at least two layers of different transparent media.  This is done for two primary reasons:  to protect the reflecting edge of the inner layer from chipping, and to reduce the size of the cone of light that will be trapped in the fiber. The first reason is discussed below; the second is discussed on a future page
 

If the edge of the glass tube becomes scratched or chipped, the normal to the edge will no longer be uniform.  Thus different parts of the beam traveling through the pipe will strike the edge at different angles of incidence.  Not only will this scatter the beam inside the tube due to diffuse reflection, but it will result in light escaping the tube.  In Figure A to the left, you can see that the light indicated with the purple angle has an angle of incidence of 38o.  This is less than the critical angle of 42o found on the previous page, so much of the light will escape.

To protect the inner tube, called the core of the fiber, from scratches, an outer cladding coats the core.  The cladding index of refraction must be lower than that of the core if total internal reflection is to occur.  Figure B to the right illustrates how the entire beam is reflected when it strikes the smooth core surface at an angle greater than the critical angle, even when the outer surface of the fiber is uneven.

What is the meaning of a step-index fiber?  Continue to the next page to find out!