# Definitions regarding Electromagnetic waves

## Wavelength Multiplexing

The process of using different wavelengths to transmit different data just as is done in regular radio transmission is called wavelength multiplexing.

## Fresnel Reflection

It is the reflection from the boundary between materials with different refractive index, where the refraction rather than reflection is the dominant effect.

## Optical dispersion

The term dispersion means scattering widely/ in different directions. Now here when light wave is scattered in different directions than it is called optical dispersion.

We know that refractive index varies with wavelength (λ). So consequently the angle of refraction will vary with λ, leading to rainbow display.

The different colors, which have different wave lengths, have different angle of refraction as they pass through the cloud/atmosphere and are separated is called dispersion.

This relation between λ and n has a strong effect of propagation of light pulses in the optical fiber.

## Absorption

Absorption is the loss process in which the impurities take up (absorb) some of the propagation light energy. It depends on the λ of light energy.

There are two basic causes of absorption in glass.

1. One is interaction of light wave with the molecular structure of material, so called intrinsic absorption. This type of loss becomes significant for λ below 0.75µm and above 1.7µm.
2. 2nd cause of absorption is the impurities in glass. Impurities could be various metals (such as Cu, Cr etc). This absorption is highly λ dependent.

## Scattering

The process that causes light to be dispersed in all directions is called scattering.

When the light energy hits obstruction, than it scatters in all directions and most of it does not continue in the original propagation direction. So this scattered light energy is lasted. The obstructions are typically variations in the density of the material and the refractive index.

Scattering can be of two types

1. ### Rayleigh Scattering

The scattering caused by a small obstructions approximately the size of λ is called Rayleigh scattering. The losses from Raleigh scattering are related to 1/λ4. i.e. As λ increases the loss decreases as 4th power of λ.
2. ### Mie Scattering

The scattering caused by imperfections and obstructions larger than the wave lengths is called Mie scattering. This type of scattering can be minimized by improved manufacturing process.

## Electromagnetic Waves

A wave that propagates by the interchange of energy between electric and magnetic field is called electromagnetic waves.

## Group Velocity

The velocity of a group of wave length is called group velocity. It is important because most of light consists of range of wave lengths rather than a single λ.

## Photon

The packet of light energy is called photon. It is analogous to basic atomic parts (electrons, protons e.t.c)

## Polarization

Polarization mean’s “restriction of the vibrations of light waves to one direction”. In other words, the direction of the electric field of the waves determines its polarization.

## Wave length λ

The distance between successive crests of the wave is called wave length

Or

The distance of constant phase point on wave travels in one period T of sinusoidal wave is called wavelength.

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