Propagation of Waves
The process of communication involves the transmission of information from one location to another. As we have seen, modulation is used to encode the information onto a carrier wave, and may involve analog or digital methods. It is only the characteristics of the carrier wave which determine how the signal will propagate over any significant distance. This chapter describes the different ways that electromagnetic waves propagate.
Basics
An electromagnetic wave is created by a local disturbance in the electric and magnetic fields. From its origin, the wave will propagate outwards in all directions.
If the medium in which it is propagating (air for example) is the same everywhere, the wave will spread out uniformly in all directions.
Far from its origin, it will have spread out enough that it will appear have the same amplitude everywhere on the plane perpendicular to its direction of travel (in the near vicinity of the observer). This type of wave is called a plane wave. A plane wave is an idealization that allows one to think of the entire wave traveling in a single direction, instead of spreading out in all directions.
Electromagnetic Wave Propagation at the speed of light in a vacuum. In other mediums, like air or glass, the speed of propagation is slower. If the speed of light in a vacuum is given the symbol c0, and the speed in some a medium is c, we can define the index of refraction, n as:
Refraction
When the wave enters the new medium, the speed of propagation will change. In order to match the incident and transmitted wave at the boundary, the transmitted wave will change its direction of propagation. For example, if the new medium has a higher index of refraction, which means the speed of propagation is lower, the wavelength will become shorter (frequency must stay the same because of the boundary conditions). For the transmitted wave to match the incident wave at the boundary, the direction of propagation of the transmitted wave must be closer to perpendicular.
The relationship between the angles and indices of refraction is given by Snell's Law:
ni sinI = nt sint
The process of communication involves the transmission of information from one location to another. As we have seen, modulation is used to encode the information onto a carrier wave, and may involve analog or digital methods. It is only the characteristics of the carrier wave which determine how the signal will propagate over any significant distance. This chapter describes the different ways that electromagnetic waves propagate.
Basics
An electromagnetic wave is created by a local disturbance in the electric and magnetic fields. From its origin, the wave will propagate outwards in all directions.
If the medium in which it is propagating (air for example) is the same everywhere, the wave will spread out uniformly in all directions.
Far from its origin, it will have spread out enough that it will appear have the same amplitude everywhere on the plane perpendicular to its direction of travel (in the near vicinity of the observer). This type of wave is called a plane wave. A plane wave is an idealization that allows one to think of the entire wave traveling in a single direction, instead of spreading out in all directions.
Electromagnetic Wave Propagation at the speed of light in a vacuum. In other mediums, like air or glass, the speed of propagation is slower. If the speed of light in a vacuum is given the symbol c0, and the speed in some a medium is c, we can define the index of refraction, n as:
Refraction
When the wave enters the new medium, the speed of propagation will change. In order to match the incident and transmitted wave at the boundary, the transmitted wave will change its direction of propagation. For example, if the new medium has a higher index of refraction, which means the speed of propagation is lower, the wavelength will become shorter (frequency must stay the same because of the boundary conditions). For the transmitted wave to match the incident wave at the boundary, the direction of propagation of the transmitted wave must be closer to perpendicular.
The relationship between the angles and indices of refraction is given by Snell's Law:
ni sinI = nt sint
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