em wave lecture

8/2/2019 EM Wave Lecture

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Electromagnetic waves• Lecture topics

• Generation of EM waves

• Terminology

• Wave and particle models of EM radiation

• EM spectrum

Generation of EM waves• Acceleration of an electrical charge

• EM wavelength depends on length of time that the chargedparticle is accelerated

• Frequency depends on number of accelerations per second

• ‘Antennas’ of different sizes

• Nuclear disintegrations = gamma rays

• Atomic-scale antennas = UV, visible, IR radiation• Centimeter/Meter-scale antennas = radio waves

• http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35



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Oscillating electric dipoles

• There is no fundamental constraint on the frequency of EM radiation,provided an oscillator with the right natural frequency and/or an energysource with the minimum required energy is present

Water molecule

Electric dipole: separation of positive and negative charges( permanent or induced )

Electromagnetic Spectrum

• EM Spectrum

• Continuous range of EM radiation

• From very short wavelengths (<300x10-9 m)

• High energy

• To very long wavelengths (cm, m, km)

• Low energy

• Energy is related to wavelength (and hencefrequency)



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EM wave terminology

• EM waves characterized by:

• Wavelength, λ (m)

• Amplitude, A (m)

• Velocity, v or c (m s-1)

• Frequency, f or ν (s-1 or Hz) – cycles per second

• Sometimes period, T (timefor one oscillation i.e., 1/f)

v

Wavelength units

• EM wavelength λ specified using various units

• cm (10-2 m)

• mm (10-3 m)

• micron or micrometer , µm (10-6 m)

• nanometer, nm (10-9 m)

• Angstrom, Å (10-10 m, mostly used in astronomy)

• f (or ν) is waves/second, s-1

or Hertz (Hz) – also MHz, GHz• Wavenumber (inverse wavelength) also commonly used:given by 1/λ (sometimes also 2π/λ) e.g. cm-1

(symbol: )

• What is the wavenumber (in cm-1) equivalent to λ = 1 µm?

ν



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Electromagnetic energy

• EM radiation defined by wavelength (λ), frequency (f) and velocity (v) where:

v = f λ

• i.e. longer wavelengths have lower frequencies etc.

• v and λ can change according to medium – f is constant

• Generally more useful to think in terms of λ (numbers are easier)

• NB. Where v = c, this relationship refers to wavelength in a vacuum.

Digression – radio waves

• Why is FM radio higher quality than AM radio?

AM FM



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Digression – radio waves

• Why is FM radio higher quality than AM radio?

• AM = Amplitude modulation (530 – 1700 kHz)

• FM = Frequency modulation (87.8 – 108 MHz)

• EM wave amplitude can be affected by many things – passingunder a bridge, re-orienting the antenna etc.

• No natural processes change the frequency

• Radiation with frequency f will always have that frequency until it is absorbed and converted into another form of energy

Wave phase and angular frequency

• Angular frequency ω = 2πf = 2π/T

• Frequency with which phase changes

• Angles in radians (rad)• 360° = 2π rad, so 1 rad = 360/2π =57.3°

• Rad to deg. (*180/π) and deg. torad (* π/180)



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Light is not only a wave, but also a particleNewton proposed wave theory of light (EMR) in 1666:observation of light separating into spectrum

The Photoelectric Effect (H. Hertz [1887], A. Einstein[1905]) – visible light incident on sodium metal

Posed problems if light was just a wave:The electrons were emitted immediately (no time lag)

Increasing the intensity of the light source increased

the number of electrons emitted but not their energy

Red light did not cause any electrons to be emitted, at

any intensity

Weak violet light ejected fewer electrons, but with

greater energy

Max Planck (1900) found that electron energy was proportional to the frequency of theincident light

Wave-particle duality

Property of EM radiation Consistent with WAVE PARTICLE

Reflection Yes Yes

Refraction Yes Yes

Interference Yes No

Diffraction Yes No

Polarization Yes No

Photoelectric effect No Yes



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Photons

The energy of a single photon is: hf or = (h/2π)ω

where h is Planck's constant, 6.626 x 10-34 Joule-seconds

One photon of visible light contains about 10-19 Joules - not much

Φ is the photon flux, or the number of photonsper unit time in a beam.

Where P is beam power.

ω

Φ =P

hv=Pλ

hc

Particle model of radiation• EMR intimately related to atomic structure and energy

• Atom: +ve charged nucleus (protons+neutrons) & -vecharged electrons bound in orbits

• Electron orbits are fixed at certain levels, each levelcorresponding to a particular electron energy

• Change of orbit either requires energy (work done), or releasesenergy

• Minimum energy required to move electron up a full energy level

(can’t have shift of 1/2 an energy level)• Once shifted to a higher energy state from the ground state, theatom is excited , and possesses potential energy

• Released as electron falls back to lower energy level



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Particle model of radiation

Bohr quantized shell model of the atom (1913): electrons jump from one orbit toanother only by emitting or absorbing energy in fixed quanta (levels)

If an electron jumps one orbit closer to the nucleus, it must emit energy equal to thedifference of the energies of the two orbits. When the electron jumps to a larger orbit, itmust absorb a quantum of light equal in energy to the difference in orbits.

Particle model of radiation: atomic shells

Electron energy levels are unevenly spaced and characteristic of aparticular element. This is the basis of spectroscopy.To be absorbed, the energy of a photon must match one of the

allowable energy levels in an atom or molecule.



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• EM radiation also considered in quantum terms, where each

photon carries an energy E (in Joules) given by:E = hf (or h ν)

• where h is Planck’s constant (6.626x10-34 J s), f = frequency


• Combining the two relations we have:

• i.e. the energy of a photon is inversely proportional to λ

• Implications for sensor design, pixel size etc.

E =hv

λ



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Frequency decomposition

• Naturally occurring EM radiation hardly ever consists of a single frequency or wavelength

• But, any arbitrary EM fluctuation can be thoughtof as a composite of a number (potentially infinite)of different ‘pure’ periodic functions

• This is known as Fourier decomposition

• So any EM wave can be regarded as a mixtureof pure sine waves with differing frequencies, and

the propagation of each frequency component can

be tracked completely separately from the others.

• In remote sensing, the implication is thatindividual frequencies can be consideredindividually, then the results summed over allrelevant frequencies.

Broadband vs. Monochromatic

• EM radiation composed entirely of a single frequency is termedmonochromatic (‘one color’)

• Radiation that consists of a mixture of frequencies is called broadband .

• So transport of broadband radiation can always be understood in termsof the transport of individual constituent frequencies (monochromaticradiation)

Plane waves have onlyone frequency, ω.

This light wave has manyfrequencies. And thefrequency increases intime (from red to blue).

L i g h t e l e c t r i c

f i e l d

Time



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Photochemistry

Many chemical reactions that take place in the atmosphere, includingthose that produce smog, are driven by sunlight.

The stratospheric ozone layer also owes its existence to photochemical

processes that break down oxygen molecules (O2).

The photon energy E = h ν is acrucial factor in determining whichfrequencies of EM radiationparticipate in these processes.

Production of tropospheric ozone (a major pollutant)

Requires λ < 0.4 µm (i.e., sunlight)

The Electromagnetic Spectrum

What wavelengths are associated with sunburn?

The EM spectrum is subdivided into a fewdiscrete spectral bands.

EM radiation spans an enormous range of frequencies; the bands shown here arethose most often used for remote sensing.

Boundaries between bands are arbitraryand have no physical significance, exceptfor the visible band.

Note that the ‘visible’ band is subjective –some insects can see ultraviolet light!



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The Ultraviolet (UV)

The UV is usually broken up into three regions, UV-A (320-400nm), UV-B (290-320 nm), and UV-C (220-290 nm).

UV-C is almost completely absorbed by the atmosphere. You canget skin cancer even from UV-A.

Remote sensing of ozone (O3) uses UV radiation.

Reaches surface;Relatively harmless; Stimulates fluorescence insome materials

Mostly absorbed by O3 in stratosphere; smallfraction (0.31-0.32 µm) reaches surface andcauses sunburn (effect of ozone depletion?);energetic enough for photochemistry

Photodissociates O2 and O3; absorbedbetween 30 and 60 km

Visible lightWavelengths and frequenciesof visible light (VIS) Atmosphere mostly transparent – optical remote

sensing techniques, surface mapping etc.

• Includes wavelength of peakemission of radiation by the Sun(~50% of solar output in thisrange)• Cloud-free atmosphere mostly

transparent to VIS wavelengths,so most are absorbed at theEarth’s surface• Clouds are highly reflective inthe VIS – implications for climate?



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The Infrared (IR)• Sub-mm wavelengths

• Unimportant for atmosphericphotochemistry – why?

• IR regions subdivided by wavelength and/or source of radiation

– Region just longer than visible knownas near-IR, NIR (0.7 – 4 µm) - partially

absorbed, mainly by water vapor

– Reflective (shortwave IR, SWIR)

– Emissive or thermal IR (TIR; 4 – 50µm) – absorbed and emitted by water

vapor, carbon dioxide, ozone and other trace gases; important for remote sensing

and climate

– Far IR (0.05 – 1 mm) – absorbed by

water vapor ; not widely exploited Note boundary (~4 µm) – separatesshortwave and longwave radiation

The microwave (µ-wave) region

• RADAR

• mm to cm wavelengths

• Usually specified as frequency,not wavelength

• Various bands used by RADARinstruments

• Long λ so low energy, hencerequire own energy source(active microwave)

• Penetrates clouds, planetary

atmospheres – useful for mapping

• Weather – monitor rainfall,tornadoes, t-storms etc.



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The electromagnetic spectrum

Now, we’ll run through the entire electromagnetic spectrum, starting atvery low frequencies and ending with the highest-frequency gamma rays.

The transition wavelengths are a bit arbitrary…

60-Hz radiation from

power lines

This very-low-frequency currentemits 60-Hz electromagnetic waves.

No, it is not harmful. A flawed epide-miological study in 1979 claimedotherwise, but no other study hasever found such results.

Also, electrical power generation has increased exponentiallysince 1900; cancer incidence has remained essentially constant.

Also, the 60-Hz electrical fields reaching the body are small;they’re greatly reduced inside the body because it’s conducting;and the body’s own electrical fields (nerve impulses) are muchgreater.



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Long-wavelength

EM spectrum

Arecibo radiotelescope

Radio & microwave regions (3 kHz – 300 GHz)



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• Consists of 24 orbiting satellites in “half-synchronous orbits” (tworevolutions per day).

• Four satellites per orbit,equally spaced, inclinedat 55 degrees to equator.

• Operates at 1.575 GHz(1.228 GHz is a referenceto compensate for atmos-pheric water effects)

• 4 signals are required;

one for time, three for position.

• 2-m accuracy

Global positioning system (GPS)

Microwave ovens

Microwave ovens operate at 2.45 GHz,where water absorbs very well.

Percy LeBaronSpencer, Inventor

of the microwaveoven



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22,300 miles (36,000 km) above the earth’s surface

6 GHz uplink, 4 GHz downlink

Each satellite is actually two (one is a spare)

Geosynchronous communications satellites

Cosmicmicrowavebackground

• Interestingly,blackbody radiationretains a blackbodyspectrum despitethe expansion of theuniverse. It does getcolder, however.

The cosmic microwavebackground is blackbody

radiation left over from

the Big Bang

Wavenumber (cm-1)

Peak frequency is ~ 150 GHz

Microwave background vs. angle



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IR is useful for

measuring thetemperature of objects.

Old Faithful

Hotter andhence brighter

in the IR

InfraredLie-detection



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The military uses IR to see objects it

considers relevant

IR light penetrates fog and smoke better than visible light.

The infrared space observatory

Stars that are justforming emit lightmainly in the IR.



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Using mid-IR laser light

to shoot down missiles

The Tactical High Energy Laser uses a high-energy,deuterium fluoride chemical laser to shoot downshort range unguided (ballistic flying) rockets.

Wavelength =3.6 to 4.2 µm

Laser welding

Near-IR wavelengths arecommonly used.

Laser pointer (red)



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Auroras

Auroras are due tofluorescence from

molecules excited bythese charged particles.

Different colors are fromdifferent atoms and

molecules.

O: 558, 630, 636 nm

N2+: 391, 428 nm

H: 486, 656 nm

Solar wind particles spiral around the earth’smagnetic field lines and collide with atmos-

pheric molecules, electronically exciting them.

Fluorescent lights

Use phosphors (transition metal compounds that exhibit phosporescencewhen exposed to UV light)

“Incandescent” lights (normal light bulbs) lack the emission lines



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The eye’s response to light and color

The eye’s cones have three receptors, one for red, another for green, and a third for blue.

The eye is poor at distinguishing spectraBecause the eye perceives intermediate colors, such as orange andyellow, by comparing relative responses of two or more differentreceptors, the eye cannot distinguish between many spectra.

The various yellow spectra below appear the same (yellow), and thecombination of red and green also looks yellow



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UV from the sun

The ozone layer absorbs wavelengths less than 320 nm (UV-B andUV-C), and clouds scatter what isn’t absorbed.

But much UV (mostly UV-A, but some UV-B) penetrates theatmosphere anyway.

IR, Visible, and UV Light and Humans

(Sunburn)

We’re opaque in the UV and visible, but not necessarily in the IR.

Skin

surface



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Flowers in the UV

Since bees see in the UV (they have a receptor peaking at 345 nm),flowers often have UV patterns that are invisible in the visible.

Visible UV (false color)

Arnica angustifolia Vahl

The sun in the UV

Image takenthrough a

171-nm filter by NASA’s

SOHOsatellite.



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The very short-wavelength regions

Soft x-rays

5 nm > λ > 0.5 nmStrongly interacts with core

electrons in materials

Vacuum-ultraviolet (VUV) "180 nm > λ > 50 nm "

Absorbed by <<1 mm of air"Ionizing to many materials"

Extreme-ultraviolet (XUV or EUV)"50 nm > λ > 5 nm"

Ionizing radiation to all materials"

EUV AstronomyThe solar corona is very hot (30,000,000 degrees K) and so emitslight in the EUV region.

EUV astronomy requires satellites because the earth’s atmosphere ishighly absorbing at these wavelengths.



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The sun also emits x-rays

The sun seen in the x-ray region

Matter falling into a black hole emits x-rays

A black hole accelerates particles to very high speeds

Black hole

Nearby star



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Supernovas emit x-rays, even afterward

A supernovaremnant in anearby galaxy (theSmall MagellanicCloud).

The false colorsshow what thissupernovaremnant looks likein the x-ray (blue),visible (green) andradio (red) regions.

X-rays are occasionally seen in auroras

On April 7th 1997, amassive solar stormejected a cloud of energetic particlestoward planet Earth.

The “plasma cloud” grazed the Earth,and its high energy particles created amassive geomagnetic storm.



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Atomic structure and x-rays

Ionization energy~ .01 – 1 e.v.

Ionization energy~ 100 – 1000 e.v.

X-rays penetrate tissue and do notscatter much

Roentgen’s x-ray imageof his wife’s hand (andwedding ring)



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X-rays for photo-lithography

You can only focus light toa spot size of the lightwavelength. So x-rays arenecessary for integrated-circuit applications withstructure a small fractionof a micron.

1 keV photons from asynchrotron:

2 micron lines over a baseof 0.5 micron lines.

Gamma rays result from matter-

antimatter annihilation

e-

e+

An electron and positron self-annihilate, creating two gammarays whose energy is equal to the electron mass energy, mec

2.

hν = 511 kev

More massive particles create even more energetic gammarays. Gamma rays are also created in nuclear decay, nuclear reactions and explosions, pulsars, black holes, andsupernova explosions.



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Gamma-ray bursts emit massive

amounts of gamma rays

In 10 seconds, they can emit more energy than our sun will in itsentire lifetime. Fortunately, there don’t seem to be any in our galaxy.

A new oneappears almostevery day, andit persists for ~1 second to~1 minute.

They’reprobablysupernovas.

The gamma-ray sky

Gamma Ray

The universe in

different spectralregions…

X-Ray

Visible



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Microwave

The universe in more spectral

regions…

IR

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