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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