 Optical Networking: Fundamentals of Light

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Wave Behavior

When waves move through a medium, they exhibit certain key characteristics—key because they have implications for the optical system. These characteristics cover reflection, refraction, interference, and diffraction.

Start with the most commonly known characteristic, reflection. When waves hit a surface, they bounce back. No surprise there. Look into a mirror and you can see reflection at work. In fact, nearly all objects reflect some light. The color that we see is light reflected off an object.

When light strikes a surface, it bounces off at a particular angle. This angle, the angle of reflection, is equal to the angle at which the ray of light struck the object as measured from the normal—an imaginary perpendicular line crossing the point of intersection between the ray and the surface (see Figure 3.10). Figure 3.10 The angle of incidence (a1) equals the angle of reflection (a2)

Normality—Who Needs It, Anyway?

At first glance, the normal—the imaginary perpendicular line used as a reference for calculating reflection and refraction—seems extraneous. Why not just measure the angle from the surface of the object being struck? The problem becomes what to do when light strikes a substance that's not flat, such as a curved sheet of glass. Then there's no common reference surface to work from. Hence the need for normality.

Reflection is of two types. In specular reflection, parallel light rays strike a surface and reflect off in parallel. As we'll see, specular reflection is important in understanding how waves propagate down a fiber. Diffuse reflection occurs when parallel rays are reflected off a rough surface at different angles, causing distortion. When fibers are crimped and microbends in the fiber are introduced, diffuse reflection becomes a major problem.