Electromagnetic waves
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/electromagnetic/index.html
Electromagnetic radiation, the larger family of wave-like phenomena to which visible light belongs (also known as radiant energy), is the
primary vehicle transporting energy through the vast reaches of the universe. This interactive tutorial explores the classical representation
of an electromagnetic wave as a sine function, and enables the visitor to vary amplitude and wavelength to demonstrate how this function
appears in three dimensions.
The tutorial initializes with a sine function simulating electromagnetic wave propagation traversing from left to right across the window. The
oscillating electric field vectors of the virtual electromagnetic wave are represented by blue lines, while the magnetic field vectors are
depicted in red. In order to operate the tutorial, use the mouse cursor to drag the wave back and forth in the window to observe how it
appears from different angles. The Filled slider can be employed to vary the density of vector lines appearing within the sine function, and
the Amplitude slider increases or decreases vector amplitude. Placing a checkmark in the Show Wave Color check box changes to the
wave simulate the color matching the current Wavelength slider value. This slider can be utilized to alter the wavelength of the virtual wave
between a range of 300 nanometers (ultraviolet) to 800 nanometers (infrared). As the Wavelength slider is translated, the color
corresponding to the current wavelength is acquired by the virtual electromagnetic wave (provided the Show Wave Color check box is
active), and the name (red, yellow, green, etc.) also appears above the slider bar.
An electromagnetic wave travels or propagates in a direction that is oriented at right angles to the vibrations of both the electric (E) and
magnetic (B) oscillating field vectors, transporting energy from the radiation source to an undetermined final destination. The two oscillating
energy fields are mutually perpendicular (illustrated in Figure 1) and vibrate in phase following the mathematical form of a sine wave.
Electric and magnetic field vectors are not only perpendicular to each other, but are also perpendicular to the direction of wave propagation.
By convention, and to simplify illustrations, the vectors representing the electric and magnetic oscillating fields of electromagnetic waves
are often omitted, although they are understood to still exist.
Whether taking the form of a signal transmitted to a radio from the broadcast station, heat radiating from a fireplace, the dentist's X-rays
producing images of teeth, or the visible and ultraviolet light emanating from the sun, the various categories of electromagnetic radiation all
share identical and fundamental wave-like properties. Every category of electromagnetic radiation, including visible light, oscillates in a
periodic fashion with peaks and valleys (or troughs), and displays a characteristic amplitude, wavelength, and frequency that together
define the direction, energy, and intensity of the radiation.
The classical schematic diagram of an electromagnetic wave presented in Figure 1 illustrates the sinusoidal nature of oscillating electric
and magnetic component vectors as they propagate through space. As a matter of convenience, most illustrations depicting
electromagnetic radiation purposely omit the magnetic component, instead representing only the electric field vector as a sine wave in a
two-dimensional graphical plot having defined x and y coordinates. By convention, the y component of the sine wave indicates the
amplitude of the electric (or magnetic field), while the x component represents time, the distance traveled, or the phase relationship with
another sine wave.
A standard measure of all electromagnetic radiation is the magnitude of the wavelength (in a vacuum), which is usually stated in units of
nanometers (one-thousandth of a micrometer) for the visible light portion of the spectrum. The wavelength is defined as the distance
between two successive peaks (or valleys) of the waveform (see Figure 1). The corresponding frequency of the radiated wave, which is
the number of sinusoidal cycles (oscillations or complete wavelengths) that pass a given point per second, is proportional to the reciprocal
of the wavelength. Thus, longer wavelengths correspond to lower frequency radiation and shorter wavelengths correspond to higher
frequency radiation. Frequency is usually expressed in quantities of hertz (Hz) or cycles per second (cps).
When electrons move, they create a magnetic field. When electrons move back and
forth or oscillate, their electric and magnetic fields change together, forming an
electromagnetic wave. This oscillation can come from atoms being heated and thus
moving about rapidly or from alternating current (AC) electricity.
Electromagnetic waves
http://www.walter-fendt.de/ph14e/emwave.htm
Electromagnetic Waves: Origin and Theory
Electromagnetism is defined as the combinations of alternating electric and magnetic fields created by accelerated charges that propagate out from these charges
at the speed of light in the form of waves- electromagnetic waves or radiation. Earths environment is widely affected by various types of radiation- power waves,
radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. A brief look into the origin and theory of Electromagnetic waves.
Origin: The phenomena associated with electricity and magnetism was studied over most of the nineteenth century. But the knowledge that the two fields
were interdependent began with the fantastic discovery by Hans Christian Orsted in the early 1820’s. He learnt that magnetism is ultimately caused by
moving electric charges or current, when he observed a magnetic compass needle to react to a current flowing through a wire placed near it.
Later on the simultaneous though separate discoveries made by Michael Faraday and Joseph Henry concerning electromagnetic induction in the
1830’s led to the theory of James Clerk Maxwell, which united electricity, magnetism and optics into one grand theory of light : the explanation of
electromagnetic waves.
Maxwell published his work Treastise on Electricity and Magnetism (1873), in which he showed that four fundamental mathematical equations
described the entire known electric and magnetic phenomenon. The first equation is Gauss’s law for electricity, which states that positive and negative
charges create magnetic fields; Gauss’s law for magnetism states that currents create magnetic field, which have associated north and south poles,
but single poles (monopoles) do not exist; Ampere’s law states that time varying magnetic fields induce time varying electric fields; and faraday’s law
of induction states that time varying electric fields create time varying magnetic fields. Additionally, Maxwell’s equation predicted the existence of
combined, changing electric and magnetic fields in the form of waves that traveled with the speed of light i.e. electromagnetic waves. He speculated
that accelerated charges ultimately create these electromagnetic waves, that they should exist over a wide range of frequencies and wavelengths, that
they traveled at the speed of light in a vacuum, and that they exhibited all the optical properties of visible light, such as reflection, refraction and
diffraction.
Heinrich Rudolf Hertz in 1887 verified Maxwell’s theory experimentally ten years after his death. Hertz built an induction coil device, which was
essentially a step up transformer whose high output voltage caused, sparks to jump back and forth across an air gap between two metal plates. One
wire, bent so that it too had an air gap between its ends, was placed near another wire. Hertz noticed sparks jumping across the ends of this wire at
the same frequency as the induction coil’s sparks. He concluded that electromagnetic waves propagated through air from the coil to the bent wire.
These waves proved to be radio waves of about 1 meter in wavelength. He demonstrated that the waves exhibited all the usual properties of light;
namely, they reflected, focused on parabolic mirrors, and refracted through glass. He caused them to interfere, setting up a standing wave pattern that
enabled him to calculate their speed to be the speed of light. Later experiments demonstrated that a wide range of electromagnetic wavelengths and
frequencies exist and led to the technologies of radio, television, radar and myriad other technologies important to society.
Theory: Many natural phenomena exhibit wavelike behavior. Water waves, earthquake waves, and sound waves all require a medium or substance through
which to propagate. These are examples of mechanical waves. Light can also be described as waves- waves of changing electric and magnetic fields
that propagate outward from their sources. These electromagnetic waves however do not require a medium. They propagate at 3,000,000,00 meters
per second through vacuum. Electromagnetic waves are transverse waves. In simpler terms, the changing electric and magnetic fields oscillate
perpendicular to each other and to the direction of the propagating waves.
The best source of electromagnetic waves is accelerated waves. An accelerated charge is one that is increasing or decreasing its speed or changing
its direction of motion or both. Let us imagine two charges at rest in the vicinity of each other. They are immersed in each others electric force field. If
one charge suddenly begins to oscillate up and down, the second charge experiences the change in the field of the first charge after some very small
finite time elapses. The oscillating charge was accelerated. The moving charge’s electric fields change, as do their magnetic fields. These changing
electric and magnetic fields generate each other through Faraday’s law of induction and Ampere’s law. These changing fields dissociate from the
oscillating charge and propagate out into space at the speed of light.
All periodic waves, whether they are electromagnetic or mechanical, are characterized by such properties as wave length, frequency, and speed. For
electromagnetic waves, wavelength measures the distance between the successive pulses of electric or magnetic fields. A waves’ frequency
represents how many wave pulses pass by a given point each second and is measured in cycles per second or waves per second and is measured in
cycles per second or waves per second. One wave per second is called one Hertz. Electromagnetic waves travel at the speed of light in vacuum, but
they travel more slowly when they pass through various media such as air, glass, and water. A relationship among frequency, wavelength and speed
exists for electromagnetic waves; the product of frequency and wavelength equals the speed of light. Thus, wavelength and frequency are inversely
related. The longer the frequency lower is the wavelength and vice versa.
An entire spectrum of electromagnetic waves exists, which ranges from very low frequency wavelength (power waves) to very high wavelength
(gamma rays). All wavelengths are collectively referred to as electromagnetic wavelengths and not merely the narrow range of wavelengths and
frequencies identified as visible light.
The wave nature of light describes many aspects of its behavior. Nevertheless, radiation also has its particle like characteristics. Rather than infinite or
nearly infinite series of electromagnetic waves emanating from some accelerated charge, light also appears to come in particle –like bursts of energy.
These individual bursts of energy or quanta are called photons. Each photon possesses an amount of energy that directly depends on the frequency
of the associated electromagnetic wave. Doubling the frequency of the photon of radiation doubles its energy. Thus, all types of electromagnetic
waves, photons of power waves possess the least energy and gamma-ray photons possess the greatest energy.
Since life on earth is bathed constantly in all forms of electromagnetic radiation, scientists must be aware of the potential risks, as well as benefits of
exposures to electromagnetic waves.
Antenna: Real-life Examples
Let's say that you are trying to build a radio tower for radio station 680 AM. It is transmitting a sine wave with a frequency of 680,000 hertz.
In one cycle of the sine wave, the transmitter is going to move electrons in the antenna in one direction, switch and pull them back, switch
and push them out and switch and move them back again. In other words, the electrons will change direction four times during one cycle of
the sine wave. If the transmitter is running at 680,000 hertz, that means that every cycle completes in (1/680,000) 0.00000147 seconds.
One quarter of that is 0.0000003675 seconds. At the speed of light, electrons can travel 0.0684 miles (0.11 km) in 0.0000003675 seconds.
That means the optimal antenna size for the transmitter at 680,000 hertz is about 361 feet (110 meters). So AM radio stations need very tall
towers. For a cell phone working at 900,000,000 (900 MHz), on the other hand, the optimum antenna size is about 8.3 cm or 3 inches. This
is why cell phones can have such short antennas.
You might have noticed that the AM radio antenna in your car is not 300 feet long -- it is only a couple of feet long. If you made the antenna
longer it would receive better, but AM stations are so strong in cities that it doesn't really matter if your antenna is the optimal length.
You might wonder why, when a radio transmitter transmits something, radio waves want to propagate through space away from the
antenna at the speed of light. Why can radio waves travel millions of miles? Why doesn't the antenna just have a magnetic field around it,
close to the antenna, as you see with a wire attached to a battery? One simple way to think about it is this: When current enters the
antenna, it does create a magnetic field around the antenna. We have also seen that the magnetic field will create an electric field (voltage
and current) in another wire placed close to the transmitter. It turns out that, in space, the magnetic field created by the antenna induces an
electric field in space. This electric field in turn induces another magnetic field in space, which induces another electric field, which induces
another magnetic field, and so on. These electric and magnetic fields (electromagnetic fields) induce each other in space at the speed of
light, traveling outward away from the antenna.
For more information on radio and related topics, check out the links on the next page.
Common frequency bands include the following:
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AM radio - 535 kilohertz to 1.7 megahertz
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Short wave radio - bands from 5.9 megahertz to 26.1 megahertz
Citizens band (CB) radio - 26.96 megahertz to 27.41 megahertz
Television stations - 54 to 88 megahertz for channels 2 through 6
FM radio - 88 megahertz to 108 megahertz
Television stations - 174 to 220 megahertz for channels 7 through 13
What is funny is that every wireless technology you can imagine has its own little band. There are hundreds of them! For example:
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Garage door openers, alarm systems, etc. - Around 40 megahertz
Standard cordless phones: Bands from 40 to 50 megahertz
Baby monitors: 49 megahertz
Radio controlled airplanes: Around 72 megahertz, which is different from...
Radio controlled cars: Around 75 megahertz
Wildlife tracking collars: 215 to 220 megahertz
MIR space station: 145 megahertz and 437 megahertz
Cell phones: 824 to 849 megahertz
New 900-MHz cordless phones: Obviously around 900 megahertz!
Air traffic control radar: 960 to 1,215 megahertz
Global Positioning System: 1,227 and 1,575 megahertz
Deep space radio communications: 2290 megahertz to 2300 megahertz