Electromagnetic spectrum - Wikipedia
Radio signals exist as a form of electromagnetic wave. waves proved these facts, and they were used to establish a link between radio waves and light rays. Medium wavelength radio waves are reflected from the ionosphere so they can be used for long distance communication, but not for communicating with satellites above the ionosphere. For example, there is a correlation between increased sales of ice cream and Copyright © - Revision World Networks Ltd. Electromagnetic waves require no medium through which to travel and thus can . The relationship between loudness and amplitude is not a simple one. .. Clearly, communication systems using optical fibres to carry visible light signals.
In a liquid, where the molecules are much closer together than in a gas, the vibrations are passed on more quickly.
An individual molecule does not have to move as far to push its neighbour.
Communication Systems/Wave Propagation
So, the speed of sound is higher in a liquid than in a gas and by the same argument, higher still in a solid. The speed of sound waves in various media can be related to the density and elasticity of the medium.
In general, the speed of sound in a medium is given by: Clearly, the equation above suggests that sound travels fastest in media which are least compressible and which have low density. At first glance this statement seems to disagree with our assertion that sound travels faster in solids than in liquids and faster in liquids than in gases, since in general gases have lower densities than liquids and liquids have lower densities than solids.
This apparent contradiction is resolved when we examine the relative elasticity values of the different states of matter. In general, solids have much higher values of elasticity than liquids, which in turn have much higher values of elasticity than gases. In fact, it is the elasticity values that have the main effect on the speed of sound in materials.
Thus, the above equation supports our assertion that sound travels faster in solids than in liquids and faster in liquids than in gases. The equation shows, however, that where two materials have similar elasticity values, the relative densities of the two materials will determine which material provides the higher speed of sound.
For example, sound travels almost three times as fast in helium than it does in air at the same temperature mainly due to the much lower density of helium compared to air. When the medium under consideration is air, it is possible to express E in terms of the undisturbed air pressure, P, since it is this pressure that supplies the restoring force which returns the air molecules to their equilibrium positions.
The speed of sound in air is: The pitch of a sound how high or low it is depends on its frequency. The higher the frequency, the higher the pitch. For a sound or note of specific frequency, like that produced by a tuning fork, the pitch is the same as the frequency. The Sun emits its peak power in the visible region, although integrating the entire emission power spectrum through all wavelengths shows that the Sun emits slightly more infrared than visible light.
Visible light and near-infrared light is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. This action allows the chemical mechanisms that underlie human vision and plant photosynthesis. The light that excites the human visual system is a very small portion of the electromagnetic spectrum.
- Communications System
- Electromagnetic waves in communication systems
- Electromagnetic spectrum
A rainbow shows the optical visible part of the electromagnetic spectrum; infrared if it could be seen would be located just beyond the red side of the rainbow with ultraviolet appearing just beyond the violet end. White light is a combination of lights of different wavelengths in the visible spectrum.
If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes the eyes, this results in visual perception of the scene. The brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this insufficiently-understood psychophysical phenomenon, most people perceive a bowl of fruit.
At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and technology can also manipulate a broad range of wavelengths. Optical fiber transmits light that, although not necessarily in the visible part of the spectrum it is usually infraredcan carry information.
More about radio waves and electromagnetic radiation
The modulation is similar to that used with radio waves. Ultraviolet radiation Main article: The wavelength of UV rays is shorter than the violet end of the visible spectrum but longer than the X-ray. UV is the longest wavelength radiation whose photons are energetic enough to ionize atoms, separating electrons from them, and thus causing chemical reactions.
Short wavelength UV and the shorter wavelength radiation above it X-rays and gamma rays are called ionizing radiationand exposure to them can damage living tissue, making them a health hazard. UV can also cause many substances to glow with visible light; this is called fluorescence. At the middle range of UV, UV rays cannot ionize but can break chemical bonds, making molecules unusually reactive.
Sunburnfor example, is caused by the disruptive effects of middle range UV radiation on skin cellswhich is the main cause of skin cancer.
UV rays in the middle range can irreparably damage the complex DNA molecules in the cells producing thymine dimers making it a very potent mutagen.
However, most of the Sun's damaging UV wavelengths are absorbed by the atmosphere before they reach the surface. The higher energy shortest wavelength ranges of UV called "vacuum UV" are absorbed by nitrogen and, at longer wavelengths, by simple diatomic oxygen in the air.
However, it is not harmless and does create oxygen radicals, mutations and skin damage. See ultraviolet for more information. However, due to their higher energies, X-rays can also interact with matter by means of the Compton effect. In the diagram of medium wave propagation, c shows the space wave propagation.
Basically the technique of space wave propagation is used in bands having very high frequencies. Only space wave propagation is left which can handle frequency waves of higher frequencies. The other name of space wave propagation is line of sight propagation. There are some limitations of space wave propagation.
These waves are limited to the curvature of the earth. These waves have line of sight propagation, means their propagation is along the line of sight distance. The line of sight distance is that exact distance at which both the sender and receiver antenna are in sight of each other. So, from the above line it is clear that if we want to increase the transmission distance then this can be done by simply extending the heights of both the sender as well as the receiver antenna.
This type of propagation is used basically in radar and television communication. These waves are not reflected by the ionosphere of the earth. So, for the propagation of television signal, geostationary satellites are used.
The satellites complete the task of reflecting television signals towards earth. If we need greater transmission then we have to build extremely tall antennas.Propagation of Electromagnetic Waves-Communication System-class 12 physics subject-CBSE-IITJEE-NEET
Direct Wave[ edit ] This is generally a line of sight transmission, however, because of atmospheric refraction the range extends slightly beyond the horizon. Ground Reflected Wave[ edit ] Radio waves may strike the earth, and bounce off. The strength of the reflection depends on local conditions. The received radio signal can cancel out if the direct and reflected waves arrive with the same relative strength and o out of phase with each other.
Horizontally polarized waves are reflected with almost the same intensity but with a o phase reversal.