As we see from the figure 4, all the rays contained within the cone are accepted by the optical fiber.
Let us take two extreme rays; one at the lowest possible angle (along the axis of the fiber), and one at the highest
(b)
possible angle ( ). Take a length L along the fiber axis traveled by the rays.
(c)
Let us now transmit a narrow pulse of light. The light pulse indicates binary information. If there is a pulse then a bit is
present, otherwise the bit is absent. When the light is switched on, all the rays are switched on at the same time. The pulse energy is therefore divided between different rays which travel by different paths inside the fiber.
(d)
The pulse along the axis of the optical fiber takes less time to travel the distance L, than the pulse which travels at the
extreme angle .
( e )
As shown in the figure 4, the distance traveled by the extreme ray is .
The time difference between the axial ray and the extreme ray then is:
where c is velocity of light. Since the core material is glass, , and since , it can lie between 1 and 1.5.
are accepted by the optical fiber. 
). Take a length L along the fiber axis traveled by the rays. 
. 
. 
, and since 
, it can lie between 1 and 1.5. 
