When light encounters a boundary
between two transmissive mediums with differing index of refraction
value, it may be reflected back into the first medium at the
interface boundary, bent at a different trajectory (i.e., refracted)
as it passes into the second medium, or some combination of the two
(see Figure 9.2).
The actual result depends on the angle
the light strikes the interface (angle of incidence) and the
wavelength dependent index of refraction values for the two
materials. As the light passes from one medium to another, the
refracted light obeys Snell’s law (see Equation 9.2).
By convention, the angles used in
calculating the light paths are measured from a line drawn normal to
the axis of the core-clad boundary or fiber center line. n1sin θ1 =
n2sin θ2 Based on this relationship, as the angle of incidence (θ1)
increases, the angle of refraction (θ2) approaches 90◦.
The angle θ1 which results in θ2 =90°
is called the critical angle. For angles of incidence greater than
the critical angle, the light is essentially reflected entirely back
into the first medium at an angle equal to the angle of incidence.
This condition is called “total
internal reflection,” and it is the basic principle by which
optical fibers work. The angle of the reflected light is called the
angle of reflection.
FIGURE 9.2 Refraction of light at boundary between different mediums. (Courtesy of Corning Cable Systems LLC and corning® Inc.)
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