Download Interference Due To Electromagnetic Waves

Supreme education council
Doha,Qatar
Al Ieman Independent Girls School
Electromagnetic radiation
2009/2010
Done By:
Hind ahmed Al_Derbesti
12I
1
Table of contents:
Introduction:
In this research we will talk about different topics such like electromagnetic
radiation, magnetic field, etc….Explain electromagnetic radiation in terms of
2
oscillating electric and magnetic field and know that all electromagnetic waves
travel with the same velocity in free space. Describe the main characteristics and
applications of the different parts of the electromagnetic spectrum and give
examples of the reflection, refraction, and interference of electromagnetic waves.
Electromagnetic Radiation:
All electromagnetic radiation has fundamental properties and behaves in predictable ways
according to the basics of wave theory. Electromagnetic radiation consists of an electrical
field (E) which varies in magnitude in a direction perpendicular to the direction in which the
radiation is traveling, and a magnetic field (M) oriented at right angles to the electrical field.
Both these fields travel at the speed of light (c).
Oscillating electric:
A linear electrodynamic machine is disclosed having permanent magnet excitation and an end flux
leakage control. A flux carrying body having a plurality of annular magnets of alternating radially
oriented polarities is disposed concentrically with respect to a stator having a plurality of annular wound
coils. Relative axial reciprocation of the body and stator causes a reversal of the flux linking the coils
while the flux in the body remains substantially constant. Focusing magnets provided on each end of the
body and axial extensions provided on the stator insure uniformity of the alternating flux linking the
coils and a constant flux intensity in the body to achieve high efficiencies
Magnetic Field:
3
Magnetic fields are produced by electric currents, which can be macroscopic currents in wires, or
microscopic currents associated with electrons in atomic orbits. The magnetic field B is defined
in terms of force on moving charge in the Lorentz force law. The interaction of magnetic field
with charge leads to many practical applications. Magnetic field sources are essentially dipolar in
nature, having a north and south magnetic pole. The SI unit for magnetic field is the Tesla, which
can be seen from the magnetic part of the Lorentz force law Fmagnetic = qvB to be composed of
(Newton x second)/(Coulomb x meter). A smaller magnetic field unit is the Gauss (1 Tesla =
10,000 Gauss).
Main characteristics of electromagnetic radiation:
Wavelength and Frequency
Two characteristics of electromagnetic radiation are particularly important for understanding
remote sensing. These are the wavelength and frequency.
Wavelength and frequency are related by the following formula:
4
The wavelength is the length of one wave cycle, which can be measured as the distance between
successive wave crests. Wavelength is usually represented by the Greek letter lambda ().
Wavelength is measured in metres (m) or some factor of metres such as nanometres (nm, 10-9
metres), micrometres (m, 10-6 metres) or centimetres (cm, 10-2 metres). Frequency refers to the
number of cycles of a wave passing a fixed point per unit of time. Frequency is normally
measured in hertz (Hz), equivalent to one cycle per second, and various multiples of hertz.
Electromagnetic spectrum:
when white light is shone through a prism it is separated out into all the colours of the rainbow;
this is the visible spectrum. So white light is a mixture of all colours. Black is NOT a colour; it is
what you get when all the light is taken away.
That was easy. Some physicists pretend that light consists of tiny particles which they call
photons. They travel at the speed of light (what a surprise). The speed of light is about
300,000,000 meters per second. When they hit something they might bounce off, go right
through or get absorbed. What happens depends a bit on how much energy they have. If they
bounce off something and then go into your eye you will "see" the thing they have bounced off.
Some things like glass and perspex will let them go through; these materials are transparent.
Black objects absorb the photons so you should not be able to see black things: you will have to
think about this one. These poor old physicists get a little bit confused when they try to explain
why some photons go through a leaf, some are reflected, and some are absorbed. They say that it
is because they have different amounts of energy.
Other physicists pretend that light is made of waves. These physicists measure the length of the
waves and this helps them to explain what happens when light hits leaves. The light with the
longest wavelength (red) is absorbed by the green stuff (chlorophyll) in the leaves. So is the light
with the shortest wavelength (blue). In between these two colours there is green light, this is
allowed to pass right through or is reflected. (Indigo and violet have shorter wavelengths than
blue light.)
Examples:
5
Speed of light
Radio: Yes, this is the same kind of energy that radio
stations emit into the air for your boom box to capture
and turn into your favorite Mozart, Madonna, or Justin
Timberlake tunes. But radio waves are also emitted by
other things ... such as stars and gases in space. You
may not be able to dance to what these objects emit,
but you can use it to learn what they are made of.
Microwaves: They will cook your popcorn in just a
few minutes! Microwaves in space are used by
astronomers to learn about the structure of nearby
galaxies, and our own Milky Way!
Infrared: Our skin emits infrared light, which is why
we can be seen in the dark by someone using night
vision goggles. In space, IR light maps the dust
between stars.
Visible: Yes, this is the part that our eyes see. Visible
radiation is emitted by everything from fireflies to
light bulbs to stars ... also by fast-moving particles
hitting other particles.
Ultraviolet: We know that the Sun is a source of
ultraviolet (or UV) radiation, because it is the UV rays
that cause our skin to burn! Stars and other "hot"
objects in space emit UV radiation.
6
X-rays: Your doctor uses them to look at your bones
and your dentist to look at your teeth. Hot gases in the
Universe also emit X-rays .
Gamma-rays: Radioactive materials (some natural and
others made by man in things like nuclear power
plants) can emit gamma-rays. Big particle accelerators
that scientists use to help them understand what matter
is made of can sometimes generate gamma-rays. But
the biggest gamma-ray generator of all is the
Universe! It makes gamma radiation in all kinds of
ways
EM Spectrum:
The electromagnetic spectrum ranges from the shorter wavelengths (including gamma and xrays) to the longer wavelengths (including microwaves and broadcast radio waves). There are
several regions of the electromagnetic spectrum which are useful for remote sensing.
Visible Spectrum:
The light which our eyes can detect forms the visible spectrum. It is important to note how small
a portion of the electromagnetic spectrum is represented by the visible region.
7
Radiation Interaction with the Earth:
Radiation that is not absorbed or scattered in the atmosphere can reach and interact with the
Earth's surface. There are three (3) forms of interaction that can take place when energy strikes,
or is incident (I) upon the surface. These are: absorption (A); transmission (T); and reflection
(R).
Reflection: reflected light is what we know as color; i.e. chlorophyll in plants reflects green
light.
Absorbtion: the incident energy is not reflected or transmitted but is transformed into another
form, such as heat i.e. a rock, or absorbed by chlorophyll in the process of photosynthesis.
Transmission: when energy propagates through a medium, what is not absorbed or reflected will
be transmitted through i.e. an ultraviolet filter on a camera absorbs UV rays but allows the
remaining energy to expose the film. Changes in density can also slow the velocity resulting in
refraction such as light through a prism.
8
Radiation Interaction with the Atmosphere:
The Earth's atmosphere acts as a filter to remove radiation such as cosmic rays, gamma rays, xrays, UV rays, and large portions of the electromagnetic spectrum through the process of
absorbtion and scattering by gases, water vapor, and particulate matter (dust).
Scattering occurs when particles or large gas molecules present in the atmosphere interact with
and cause the electromagnetic radiation to be redirected from its original path. There are three (3)
types of scattering which take place: Raleigh Scatter, Mie Scatter, Non-selective Scatter.
Rayleigh Scatter occurs when particles are very small compared to the wavelength of the
radiation. These could be particles such as small specks of dust or nitrogen and oxygen
molecules. This is the cause of the blue sky; it is red in the mornings and evenings because light
has a longer path through the atmosphere and the blue wavelengths (or shorter wavelenths) are
scattered so completely that it leaves only red (the longer) wavelengths.
Mie Scattering occurs when the particles in the atmosphere are the same size as the wavelengths
being scattered. Dust, pollen, smoke and water vapour are common causes of Mie scattering
which tends to affect longer wavelengths. Mie scattering occurs mostly in the lower portions of
the atmosphere where larger particles are more abundant, and dominates when cloud conditions
are overcast.
Non-Selective Scattering occurs when the particles are much larger than the wavelength of the
radiation. Water droplets and large dust particles can cause this type of scattering and causes fog
and clouds to appear white to our eyes because blue, green, and red light are all scattered in
approximately equal quantities (blue+green+red light = white light).
9
Reflection:
Plane Wave Reflection
"The angle of incidence is equal to the angle of
reflection" is one way of stating the law of reflection
for light in a plane mirror.
Sound obeys the same law of reflection .
Refraction:
Refraction of Light
Refraction is the bending of a wave when it enters a medium where it's speed is different. The
refraction of light when it passes from a fast medium to a slow medium bends the light ray
toward the normal to the boundary between the two media. The amount of bending depends on
the indices of refraction of the two media and is described quantitatively by Snell's Law.
10
Refraction is responsible for image
formation by lenses and the eye.
As the speed of light is reduced in the slower medium, the wavelength is shortened
proportionately. The frequency is unchanged; it is a characteristic of the source of the light and
unaffected by medium changes.
Interference of electromagnetic waves:
Interference Due To Electromagnetic Waves
Coming now to the prevention of interference by elec tromagnetic waves such as emanate from other
wireless stations than that with which it is desired to hold communication, and to the prevention of
interference by stray electromagnet waves, we find the solution of the problem depends upon the
character of the message-bearing waves, the energy of which it is desired to convey to the receiving
device, and also upon the character of the disturbing waves, the energy of which it is desired to exclude
or divert from the receiving device.
We can control the character of the waves whose energy we wish to receive, by suitably designing the
apparatus to be used at the transmitting station, but we have no control over the character of the
disturbing waves, except in so far as these arise from wireless stations within operative range of the
receiving station.
The simplest solution of this problem is to cause each transmitter to send out its signals by means of
persistent trains of simple harmonic waves of a frequency materially different from that employed by any
other transmitter within operative range of the receiving station with which communication is to be
maintained and to make each receiver responsive only to persistent trains of simple harmonic waves of
the frequency employed by the transmitter with which it is in communication.
11
Fig. 2.
By this means the system is rendered selective and becomes a multiple system of telegraphy, permitting
he operator at each station to select the station with which he wishes to hold communication to the
exclusion of all other stations, and by which a number of messages may be transmitted simultaneously in
a given region without interfering with one another.
Since the stray electromagnetic waves arising from lightning, etc., are not persistent trains of simple
harmonic waves, but partake more of the character of isolated impulses, the receiver in such a system
does not respond to such stray electromagnetic waves and it is therefore freed from interference which
would otherwise arise from such sources.
The manner in which a transmitting station is made to develop persistent trains of simple harmonic
electromagnetic waves of one frequency to the exclusion of other frequencies, though simple in itself, in
practice requires the strictest attention to certain details, and these may be best understood by the
consideration of a concrete case, this being the manner in which such problems are usually presented to
the engineer if not to the inventor
12
Conclusion:
At the end of this research we have learned about electromagnetic radiation and
some of its parts and I would like in the end to thank my teacher for helping me to
finish this research.
References:
http://en.wikipedia.org/wiki/Electromagnetic_radiation
http://en.wikipedia.org/wiki/Electronic_oscillator
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html
http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html#c3
http://www.freepatentsonline.com/4349757.html
http://hosting.soonet.ca/eliris/remotesensing/bl130lec3.html
http://hyperphysics.phy-astr.gsu.edu/hbase/ems1.html
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
http://www.purchon.com/physics/electromagnetic.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reflec2.html
http://hyperphysics.phy-astr.gsu.edu/Hbase/geoopt/refr.html
http://chestofbooks.com/crafts/popular-mechanics/Amateur-Work-4/Interference-Due-ToElectromagnetic-Waves.html
13