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Electromagnetic Radiation

Dr. Rohit Goyal

Professor, Civil Engineering

Malaviya National Institute of Technology

Jaipur

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Topics CoveredRadiation of Energy

Maxwell Wave Model of EMR

EMR Spectrum

Stephan-Boltzman law

Wien Displacement LawQuantum (Photons) Theory

EMR Emitted by Sun

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Transfer of Energy/Heat Energy is the ability to do work

Energy can be transferred from one body to

another or one place to another by Conduction: It is transfer of heat along a solid object; it is

this process that makes the handle of a rod hot, even ifonly the tip of that rod is in the fireplace

Convection: It is transfers of heat through the exchange ofhot and cold molecules; this is the process through which

water in a kettle becomes uniformly hot even though onlythe bottom of the kettle contacts the flame

Radiation: It is transfer of heat via electromagneticradiation; this is the principal mechanism through which afireplace warms a room

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Heat Transfer

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Electromagnetic Radiation Some radiation phenomena can be described

in terms of wave theory, and others can beexplained in terms of quantum theory. Neithertheory, however, completely explains allexperimental observations. These theories are Maxwell Wave Model

Quantum Theory, also known as particle theory.French physicist Louis Victor de Broglie suggestedthat just as waves can sometimes behave asparticles, so can particles, such as electrons, behaveas waves. For this discovery about the nature ofelectrons, de Broglie is awarded the Nobel Prize inphysics in the year 1929.

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Maxwell Wave Theory Maxwell conceptualized EMR as

electromagnetic wave that travels throughspace at the speed of light ( 3 x 108 m/sec).

According to this theory wave consists of twofluctuating fields; one electric & othermagnetic, orthogonal to each other and bothare perpendicular to direction of travel of wave

The only form of wave motion that doesntrequires material medium for transmission isthe EM wave; in this case the displacement isof electric and magnetic fields of force in space

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Schematic Diagram of EMR

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Wave TheoryTwo important properties of EMR waves

are

Wavelength ( ): Mean distance betweenmaximums or minimums

Frequency ( ): Number of wavelengths thatpass a point per unit time (measured inhertz, 1 Hz= 1 cycle/sec).

c = where c is speed of light

When EMR passes from one substance toanother, the speed of light and wavelengthchanges while the frequency remains same.

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Wave Length

(a) Longer wavelength; (b) shorter wavelength.

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EMR Spectrum

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EMR Spectrum for RS

Reflective IR Emissive IR

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EMR Spectrum Wavelength interval in the EMR spectrum is

commonly referred as a band, channel or

region. Example visible (0.4-0.7 m) band iscomposed of

Blue (0.4-0.5 m)

Green (0.5-0.6 m)

Red (0.6-0.7 m)

Similarly Near IR band is usually taken as 0.7-1.3 m. MIR as 1.3-3 m and Thermal IR bandsuseful for RS are 3-5 m and 8-14 m.

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Blackbody RadiationAll objects with temperature above zero

(-273 0C or 0 K) emit EMR

Blackbody is an idealized object thatabsorbs all the radiation that strikes itssurface, without reflecting any of theradiation. It than reemits all energy

incident upon it. Amount of Energyemitted is function of temperature andwavelength and is given by PlancksRadiation Law

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Planks Radiation Law The Spectral radiant exitance (emittance) in

W/(m2 m) from a black body for specificwavelength is given by

Where h=Plancks Constant =6.2559x10-34 Js

c = velocity of light = 2.9979x108 m/s

k = Boltzmanns constant = 1.38x10-23 J/K

T = absolute temp. of body in degree Kelvin

118

5

kThc

e

hcM

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Stefan Boltzman Law If we integrate the previous equation for = 0

to then total radiant emittanceM = 8 5k4/h3c3 T4 = T4

Where is the Stefan Boltzman constant5.6697 x 10-8 W/m2K4

M is total radiant exitance from the surface ofthe body (W/m2) and T is absolutetemperature in K (Kelvin)

Sun temperature is 6000 K whereas that ofearth is 300 K. So, if both are taken asblackbodies than sun radiates about 7.35 x 107W/m2, whereas earth radiates only about 460W/m2

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Spectral Radiant Exitance Curve

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Wien Displacement Law

Dominant wavelength in which blackbody emitsenergy also shifts towards the shorterwavelength as the temperature increases.

According to Wien Displacement Law max-Energy = A/T, where A is a constant = 2898

m K and max-Energy is wavelength of maximumspectral radiant exitance in m

Therefore suns dominant wavelength is about0.483 m (Visible) and that of earth is 9.66 m(Thermal IR).

Although suns dominant wavelength is 0.48 mbut it produces a continuous spectrum from

gamma to radio waves

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Gray Body No real body is perfect emitter. Most earth

based features can be described as either graybody or selective radiator

Gray Body: These bodies radiate energy lessthan that of black body Emissivity of gray body is defined as =(Mg) /Mb,

where Mg is emittance of gray body and Mb is that ofblack body.

For gray body emissivity is constant and does notvary with wavelength ( ).

Selective radiator is body for which emissivityvaries with wavelength ( )

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Spectral Emissivity Characteristic

Curve

Blackbody

Graybody

Selective Radiator

1

0

Spectral

Emssivity

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Spectral radiant Emittance for

different type of bodies

Blackbody

Graybody

Selective RadiatorSpectral

Radiant

Emittance

(M )

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Quantum Theory Einstein concluded that when light interacts with

matter, it behaves as through it is composed ofmany individual bodies called photons withproperties like energy & momentum

In quantum theory EMR is described as discreetpackets of energy known as quanta

Energy of quantum is given by

Q = h , where Q is in Joules, h is Planks constant= 6.626 x 10-34 J sec and is frequency in Hz

Therefore shorter wavelengths have higher energylevel and better penetration capabilities

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Light Absorption and Emission

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Release of Photon When a photon, or packet of light energy, is

absorbed by an atom, the atom gains theenergy of the photon One of the atoms electrons may jump to a higher

energy level. The atom is then said to be excited

When an electron of an excited atom falls to alower energy level, the atom may emit theelectrons excess energy in the form of aphoton

The energy levels, or orbitals, of the atoms inprevious slide have been greatly simplified toillustrate these absorption and emission

processes.

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Emission from Sun If matter is heated to higher temperatures

than electron may break free and atom maybecome ionized.

If a free electron drops in to fill the vacantenergy level, then the radiation given off isunquantized and a continuous spectrum isproduced rather than a band or series of

bands, Since every encounter of free electronwith positively charged nucleus causes rapidlychanging electric and magnetic fields

Hot surface of Sun is a plasma in whichradiation of all wavelengths is produced