We have seen that rays approaching from within the cone of acceptance are successfully propagated along the fiber.
The position and the angle at which the ray strikes the core will determine the exact path taken by the ray. There are three possibilities, called the skew, meridional and the axial ray as shown in Figure below. If light enters a fiber from a practical light source, all three rays tend to occur as well as those outside of the cone of acceptance as we have just investigated.
The meridional ray enters the core and passes through its center. Thereafter, assuming the surfaces of the core are parallel, it will always be reflected to pass through the center. The meridional ray in the end view of Figure below just happens to be drawn horizontally but it could occur equally well at any other angle.
The axial ray is a particular ray that just happens to travel straight through the center of the core.