Waves are of many kinds, such as water, sound, light waves etc. But in 1873, Maxwell derived a wave form having electric and magnetic field components, revealing the wave-like nature of electric and magnetic fields. These two components had same wavelength, frequency, speed but differ in about 90 degree, in planes in which they travel. According to Maxwell, an oscillating electric field generates an oscillating magnetic field which in turn generates an oscillating electric field, and so on forming an electromagnetic wave.
Electromagnetic radiation is radiation which is not emitted continuously as in particle radiation but is emitted in discrete units or say photons that travel at the speed of light as electromagnetic waves. Photons (in a given, non-absorbing medium) travel at the same velocity, v. The physical distance in the direction of propogation over which the electric and magnetic fields of a photon make one complete oscillation is called the wavelength, , of the electromagnetic radiation. The relationship between the light velocity, wavelength, and frequency is:
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The electromagnetic nature of all photons is the same, but photons can have different frequencies. The names we give electromagnetic radiation for different wavelength and frequency ranges are listed in the electromagnetic spectrum document. The energy, E, of one photon depends on its frequency of oscillation:
E = h = hv /
where h is Planck's constant (6.62618x10-34 JÂ·s).
It is important to be able to distinguish between :
Energy & Power
Delivered by E M Radiation
The intensity determines the amount or number of these effects which occur.
The energy of EM radiation determines the effect of the radiation on materials and is the basis for understanding analytic techniques which use EM radiation.
For example, x-rays are of short wavelength and have very high energy per photon. . Radio waves are of very large wavelength and low energy. Radio waves pass through our bodies constantly with no effect.
High intensity radio waves can cause changes in the spin polarization of nuclei when the nuclei are under a very strong magnetic field and this is the basis of nuclear magnetic resonance absorption.
Radiation is separated into two categories, ionizing and non-ionizing, to denote the energy and danger of the radiation. Ionizing radiation is radiation in which an individual particle carries enough energy to ionize an atom or molecule. Corpuscular ionizing radiation consists of fast-moving, charged particles such as electrons, positrons, or small atomic nuclei. Thermal, epithermal, and fast neutrons interact with atomic nuclei creating secondary ionizing radiation and are called indirectly ionizing radiation. Electromagnetic ionizing radiation includes X-rays and gamma-rays. Ultraviolet light also can ionize an atom or molecule, but is referred to usually as non-ionizing radiation.
The amount of ionizing radiation, or 'absorbed dose' is measured in grays. One gray (Gy) is one joule of the energy deposited per kilogram of mass.
Some types of radiation, such as neutrons or alpha particles, are more biologically damaging than photons or fast electrons when the absorbed dose from both is equal. To estimate this, dose equivalent, in a unit called the sievert (Sv), is used. Regardless of the type of radiation, one sievert of radiation produces the same biological effect. High radiation doses tend to kill cells, while low doses tend to damage or alter the genetic code of irradiated cells. The effect of very low doses is a subject of current debate.
Features of Electromagnetic Waves:
EM waves are described using a sin function in time and space. That is, at a fixed time the wave varies in Amplitude, A, with spatial position and for a given position in space the wave varies in amplitude with time. This is true of both the electric field vector, E, and the magnetic field vector, B.
E = A sin[2Ï€x/Î» -2Ï€nt]
B = B0 cos[2Ï€x/Î» -2Ï€nt] Out of phase with E by 90Â°
E and A have direction. Î» is the wavelength, Î½ is the frequency.
A special property of EM radiation is that it displays a fixed speed in vacuum, c, and fixed velocities in other uniform media so conversion from frequency to wavelengthis direct:
Î½ = c / Î»
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When light passes through other media, it's velocity decreases. Since the energy of a photon is fixed, the frequency of a photon does not change. Thus for a given frequency of light, the wavelength must decrease as the velocity decreases.
The decrease in velocity is quantitated by the refractive index, n, which is the ratio of c to the velocity of light in another medium, v:
n = c / v
We can specify a type of EM radiation by specifying it's wavelength, frequency, or wavenumber, k:
K = 1 / Î»
EM radiation also displays a particle characteristic through the concept of the photon which is a particle of no mass. This allows a means to describe features of EM radiation which are usually associated with particles such as momentum.
The energy per photon of EM radiation is related to the frequency, Î½ , of the Maxwellian sinusoidal oscillation through Planck's constant, h:
E = h Î½
That is, higher frequency is associated with higher energy. Then the Energy is given by
E = hc / Î»
Different wavelength radiations contain different amounts of energy per photon. A photon is a quantum of EM radiation that displays momentum. The momentum is expressed as:
p = h / Î» = hÎ½ / c
The brilliance, brightness, flux or intensity of a particular EM radiation is related to how many photons are delivered in a unit area per unit time. The energy of each photon is related directly to the wavelength.
3) The amplitude is related to the "intensity" of the EM radiation:
Intensity = |A|2
Intensity doesn't have direction, amplitude does.
4) Light, X-rays and other EM radiations are generally composed of a number of photons so one can consider the relationship between different waves in a beam.
i.) Coherent: If all waves in a beam are in-phase, that is have the same phase angle (peaks of waves coincide in space) they are called coherent. Waves which are not coherent can interfere with each other leading to a reduction of the intensity. For example, a laser beam is coherent while a flash light beam is incoherent. This is one reason why a laser beam can propagate over great distances while a flash light beam quickly dissipates. When considering interference of two waves one adds or subtracts amplitudes of the electric field vector E. The intensity which is measured is the square of the resulting amplitude, #3 above.
ii.) Collimated: Beams with waves which are all progressing in the same direction are termed a well collimated beam. Collimation refers to the divergence of the waves in a beam. A light bulb produces uncollimated light which spreads in all directions. The sun's rays, when they reach earth are well collimated since the angular divergence is low. A laser beam or a synchrotron x-ray beam are well collimated due to the mechanism by which the EM radiation is produced.
iii.) Monochromatic: If all waves have the same frequency (or wavelength by #2 above) they are called monochromatic (one color). A source like a light bulb, the sun, or an x-ray tube generates polychromatic (white light) radiation (many wavelengths) and a source like a laser or an x-ray synchrotron yields monochromatic radiation. The polychromacity of EM radiation is tied to the mechanism of formation. If the formation event is specific (quantum) in terms of the energy transfer associated with the formation event, monochromatic radiation results. If the formation event is statistical (distributed in energy) polychromatic radiation results.
iv.) Polarization: The electric field vector, E, for and EM wave has a direction in a plane normal to the propagation direction. If the direction is fixed relative to the propagation direction and this direction is the same for all waves in a beam, the EM radiation is said to be linearly polarized. Polarization can be produced by a number of means:
Reflection off a surface leads to linear polarization in the plane of the surface, highly birefringent materials can lead to polarization of a beam by absorption of components not with a certain polarization, some processes for formation of EM radiation produce polarized radiation laser and x-ray synchrotrons, in some cases a grating can be used to polarize radiation (Soller slits for x-rays), diffraction leads to polarized radiation depending on the geometry of diffraction. In addition to linear polarization, waves can be elliptically and circularly polarized. In circularly polarized beams the vector E rotates in direction along the propagation direction. Elliptically polarized radiation is a mixture of circularly and linearly polarized radiations, i.e. there is some rotation of the vector E but it is not symmetric. In this class we will only discuss unpolarized and linearly polarized radiation.
6) EM radiation can always be assumed to travel in a straight line.
7) EM radiation interacts with matter in different ways depending on the energy associated with a photon. That is, energy decides what happens while intensity decides how much happens. Radio waves are low energy/high wavelength (see table above) and can pass through most materials with no effect. IR vibrates bonds and can generate heat. Light changes the polarization of molecules which is a minor effect. UV can dissociate weak bonds and cause degradation. X-rays are a type of ionizing radiation that can ionize atoms and molecules. Typically, x-rays have wavelengths on the Ångstrom scale. Generally, the lower the wavelength of radiation the higher the danger due to the higher energy associated with short wavelengths see #4 above. It is also more difficult to produce and use high-energy photons since they must result from an associated high energy event and they are absorbed by most materials through the interactions mentioned above.
EFFECT OF TEMPERATURE
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The amount of electromagnetic radiation emitted by a body is directly related to its temperature. If the body is a perfect emitter (black body), the amount of radiation given off is proportional to the 4th power of its temperature as measured in Kelvin units. This natural phenomenon is described by the Stephan-Boltzmann Law. The following simple equation describes this law mathematically:
E = Ïƒ T4
According to the Stephan-Boltzmann equation, a small increase in the temperature of a radiating body results in a large amount of additional radiation being emitted.
In general, good emitters of radiation are also good absorbers of radiation at specific wavelength bands. This is especially true of gases and is responsible for the Earth's greenhouse effect. Likewise, weak emitters of radiation are also weak absorbers of radiation at specific wavelength bands. This fact is referred to as Kirchhoff's Law. Some objects in nature have almost completely perfect abilities to absorb and emit radiation. We call these objects black bodies. The radiation characteristics of the sun and the Earth are very close to being black bodies.
The wavelength of maximum emission of any body is inversely proportional to its absolute temperature. Thus, higher the temperature, shorter is the wavelength of maximum emission. This phenomenon is often called Wien's Law. The following equation describes this law:
Î»max = c/T
According to the above equation, the wavelength of maximum emission for the sun (5,800 Kelvins) is about 0.5 micrometers, while the wavelength of maximum emission for the Earth (288 Kelvins) is approximately 10.0 micrometers.
Spectrum of the sun.
The sun emits most of its radiation in a wavelength band between 0.1 and 4.0 micrometers (Âµm)
Spectrum of the Earth.
The Earth emits most of its radiation in a wavelength band between 0.5 and 30.0 micrometers (Âµm)
The graphs above illustrate two important points concerning the relationship between the temperature of a body and its emissions of electromagnetic radiation:
1. The amount of radiation emitted from a body increases exponentially with a linear rise in temperature.
2. The average wavelength of electromagnetic emissions becomes shorter with increasing temperature.
There are several types of radiations which differ according to their frequencies & wavelengths.
In between there also lies visible region, somewhere between 400nm to 700nm
The Sun produces a continuous spectrum of energy from gamma rays to radio waves that continually bathe the Earth in energy.
Represents the continuum of electromagnetic energy from extremely short wavelengths (cosmic and gamma rays) to extremely long wavelengths (microwaves).
No natural breaks in the EMS -- it is artificially separated and named as various spectral bands (divisions) for the description convenience.
Common bands in remote sensing are visible, infra-red & microwave.
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, hydrogen atoms emit radio waves of wavelength 21.12 cm.
Visible: Small portion of the EMS that humans are sensitive to:
blue (0.4-0.5 Î¼m)
green (0.5-0.6 Î¼m)
red (0.6-0.73 Î¼m)
Infrared: Three logical zones:
Near IR: reflected, can be recorded on film emulsions (0.7 - 1.3Î¼m).
Mid infrared: reflected, can be detected using electro-optical sensors (1.3 - 3.0Î¼m).
Thermal infrared: emitted, can only be detected using electro-optical sensors (3.0 - 5.0 and 8 - 14 Î¼m).
Radar sensors, wavelengths range from 1mm - 1m (Ka, Ku, X, C, S, L & P)
Analytic Techniques Using EM Radiation: (Low Energy to High Energy)
NAME Wavelength Common Name Effect
Nuclear Magnetic 1010 Radio Nuclear Spin
Electron Spin 107 TV Electron Spin
Microwave Absorption 2x105 Radar Rotate Polar
Infrared Absorption IR (Far IR) 15.4-830 IR Bond Vibrations
(Near IR) 0.7-2.5
Raman Spectroscopy IR
Elastic Light Scattering 0.4-0.7 Visible Light Changes in
Inelastic Light Scattering
UV absorption 0.01-0.4 UV-A, UV-B Move electrons
Elastic X-ray Scattering 10-4-10-2 X-Rays Ionizing
Ionization (SIMS) 10-6-10-4 Gamma-Rays Ionizing
10-9-10-6 Cosmic Rays Ionizing