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Light is a type of electromagnetic radiation that consists of particles (called photons) or waves. Photons are emitted when electrons fall from a state of higher energy to one of lower energy. Waves are caused by the electrons' vibrations.

There are two types of waves, transverse and longitudinal. Components of a transverse wave move perpendicular to the wave's motion; those of a longitudinal wave move parallel to the wave's motion.

Light is a transverse wave consisting of amplitude, wavelength, frequency, and speed. The wavelengths of the kinds of light used in optical transmission range from around 850 to 1600 nm.

Light waves have a number of characteristics. Those critical to optical networking are reflection, refraction, diffraction, and interference.

The refractive index (RI) is a measure comparing the speed of light in in a vacuum compared to the speed of light in a substance, such as glass. The higher the RI, the slower the signal travels through the substance.

When light passes from a substance of a lower to one of a higher RI, it bends. Snell's law says that at some point the light will strike the substance with the higher RI at so great an angle that it will bounce off, as if reflected from a mirror. This phenomenon is called total internal reflection.

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