TAILIEUCHUNG - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 18

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 18 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | B Page 139 Tuesday May 20 2003 2 53 PM Optical Media 139 Figure 3-18 Refraction Example Glass Diamond Air Total Internal Reflection A light ray that is being turned on and off to send data 1s and 0s into an optical fiber must stay inside the fiber until it reaches the far end. The ray must not refract into the material wrapped around the outside of the fiber because such refraction causes the loss of part of the ray s light energy. A design for the fiber that makes the outside surface of the fiber act like a mirror to the light ray moving through the fiber must be achieved. If any light ray that tries to move out through the side of the fiber is reflected back into the fiber at an angle that sends it towards the far end of the fiber this is a good pipe or wave guide for the light waves as illustrated in Figure 3-19. Figure 3-19 Total Internal Reflection The laws of reflection and refraction tell how to design a fiber that guides the light waves through the fiber with a minimum energy loss. Two conditions are needed to cause light rays in a fiber to be reflected back into the fiber with out any loss due to refraction. These two conditions are as follows The core the inside of the optical fiber has to have a larger a higher index of refraction than the material that surrounds it. The material that surrounds the core of the fiber is called the cladding. The angle of incidence of the light ray is greater than the critical angle for the core and its cladding. When both of these conditions are met all the incident light in the fiber is reflected back inside the fiber. This condition is called total internal reflection which is the foundation on which optical fiber is constructed. Total internal reflection causes the light rays in the Page 140 Tuesday May 20 2003 2 53 PM 140 Chapter 3 Networking Media fiber to bounce off the core-cladding boundary and continue their journey toward the far end of the fiber. The light follows something of a zigzag path .

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