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					  Microwaves and Light

What is light?

How does the radio “hear” the DJ downtown?

What are microwaves?

How do they boil water and heat up my soup?

How do lasers work?

How do lenses work?


Trivia question: Who is the greatest geek of all?
Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Faraday + Ampere

Changing electric and magnetic fields “feed each other” energy
E&M wave can propagate through nothing (a vacuum)
  but not through anything (a mirror)




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Spectrum of Possible Frequencies

  c = l f and c = 300,000 km/s = 186,000 mi/s
  So wavelength is l = c / f = anywhere from a mile (AM
     radios) to a nanometer (xray machines)




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Some Examples
AM radio: 535 kHz to 1600 kHz (l = 300 m)
Short wave radio: bands from 5.9 MHz to 26.1 MHz
Citizens Band (CB) radio: 26.96 MHz to 27.41 MHz
Television stations: 174-220 MHz for channels 7-13
Garage door openers, alarm systems, etc.: around 40 MHz
Baby monitors: 49 MHz
Radio controlled airplanes: around 72 MHz,
Television stations: 54-88 MHz for channels 2-6
FM radio: 88 MHz to 108 MHz (l = 3 m)
Wildlife tracking collars: 215 to 220 MHtz
MIR space station: 145 MHz and 437 MHz
New 900 MHz cordless phones: uhm … 900 MHz
Cell phones: 824 to 1800 MHz
Air Traffic Control radar: 960 to 1,215 MHz
Global Positioning System: 1,227 and 1,575 MHz
Deep space radio communications: 2290 MHz to 2300 MHz
Microwave oven: 2450MHz (l = 0.12m)
Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  How the Sunglasses Work

  Polarized light means all the electric
    fields point up and down

  Use telephone-pole molecules that
    lie parallel to each other on the
    lenses

  Only vertical electric fields get
    through between the molecules




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  What Happens in the Antenna

  Electric fields push electrons

                                                                E
  If the field is along the antenna wire,
      it moves the electrons back and
      forth along the wire: current flows

  Current can drive amplifiers that
    drive speakers, cell phones, etc

  But only if the antenna points along
    the electric field

Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  How to Make Radio Waves

  Just drive current back
     and forth through an
     antenna

  Changing magnetic field
    induces the changing
    electric field and off
    the EM wave goes




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
    Tank Circuit Oscillator
An electronic resonator swaps current in the
  inductor (K.E.) with charge stored on the
  capacitor (P.E.)
That exchange takes a characteristic period of time

Tune the period by tuning the inductance or
  capacitance

Just like a mass &
   spring oscillator




  Lecture 16, Electromagnetic waves   Phys 100, How Things Work
   How Does 100MHz Become Sound?
Audible signals f =1 kHz <<
  1000 kHz and 100 MHz so
  they must be mixed with
  the radio wave that the tank
  circuit catches.

In the case of Amplitude
   Modulation, the intensity
   of the sound appears as the
   strength of the electric
   field.



 Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Frequency Modulation

Trickier to do since the tank
   circuit must “follow” the
   radio wave frequency,
   which means a pretty fancy
   electrical circuit

But it works fine for engineers
  who are clever enough
  (viva la pocket protectors!)




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  All Waves Follow Similar Rules

  Reflection

  Refraction

  Diffraction

  Interference

  Standing or traveling



Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Metal Reflects EM Waves

Oscillating electric field impinges on metal surface
Oscillating field makes oscillating current
  which is charges moving up and down

Oscillating charges re-emit the
  same frequency electric field
  oscillations
                                                                I




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Percy Lebaron Spencer

  1945 at Raytheon – melting
    chocolate accidentally while
    monkeying around with
    microwave transmitter, then
    tried popping popcorn.

  The rest is a revolution in
    “cooking” history




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  How the Oven Makes Heat


Magnetron (2.45 GHz
microwave source)

 Metal box = resonator, which
   reflects waves back and forth

 The oven is a resonator, which stores (and builds) up
 microwave energy unless that energy is absorbed by…food!

 Sometimes use fans to stir air and scatter microwaves around
 to get even heating

Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  How the Microwave Works

  Water molecules (and fat molecules)
   are dipoles

  They align with electric fields

  Oscillating fields yank them back and
    forth creating lots of K.E.

  And K.E. in molecules is what we
    mean when we say that the shark
    fin soup is “hot”

Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  A Little More Intensity Please

  Microwave energy non-lethal weapons
                                                                Army food
                                                                  Sux!!




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Refraction Angle Depends on Frequency
 Refraction bends light at interfaces

 www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-inter-refrac.htm




                                                           QuickTime™ and a
                                                           GIF decompressor
                                                     are need ed to see this picture.




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Raindrop Refractors
Wavelength dependent bending
Descartes knew this.




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
   Color and Wavelength
Color and wavelength are different descriptions of the same
  physics
Objects reflect wavelengths/energies
  and we see the mixtures




 Lecture 16, Electromagnetic waves   Phys 100, How Things Work
   Colors and Perception
Play for yourself at:
   http://micro.magnet.fsu.edu/primer/java/primarycolors/
   additiveprimaries/index.html

Colors mix by addition or subtraction

White light has the whole rainbow
 in it (broad spectrum of wavelengths)

Photography and printing both use
  these schemes to render color


 Lecture 16, Electromagnetic waves   Phys 100, How Things Work
  Refraction Bends “Rays” of Light

  Makes microscopes and telescopes feasible
    and fish difficult to spear




Lecture 16, Electromagnetic waves   Phys 100, How Things Work
      How to Make Light Go Where You Want It

   Light is reflected/refracted/deflected at a non-planar interface
     or set of non-planar interfaces can focus at a point

   Usually just draw “rays” to represent whole (messy) waves




          QuickTime™ and a
TIFF (Uncompressed) decompressor
   are neede d to see this picture.




                                                                    Focal point


    Lecture 16, Electromagnetic waves   Phys 100, How Things Work
   Weird Cameras
Millimeter wave and micron wave images
Resolution is better for smaller wavelengths




 Lecture 16, Electromagnetic waves   Phys 100, How Things Work
    How to Make Little Bits of Light
Want to excite a lower energy electron to high energy level

Incoming photon can “knock” electron to higher level

Electron can fall back giving P.E. as energy (light)

Amount of energy is frequency of photon

   E=hf

Photon energy is all K.E.

  Lecture 16, Electromagnetic waves   Phys 100, How Things Work
   Lasers
Stimulated emission allows coherent light
   photons line up like a marching band

Light reflects in phase between mirrors
  (standing waves)

Some photons escape to be used

All at same wavelength & in phase

Lots of energy input to get this started
  (ionize all those atoms)
 Lecture 16, Electromagnetic waves   Phys 100, How Things Work
   Better Lasers
Supply high energy electrons from
  n-type reservoir in a diode
Let them fall into low energy state
  in p-type half
Various standing modes excite
Then one catches fire by using
  resonant energy transfer
  (aka stimulated emission)
  to excite LOTS of one color




                                     http://www.britneyspears.ac/lasers.htm
 Lecture 16, Electromagnetic waves     Phys 100, How Things Work
  Take home messages
 Light, microwaves, radio waves, … are all the same thing -- just
   different frequencies, wavelengths, and colors

 They all propagate at 3  108 m/s in air (or vacuum). A little
   slower in stuff.

 We generate such waves with tuned tank circuits (inductor and
  capacitor exchanging energy at a fixed frequency) or with
  clever tricks like atomic energy levels and lasers

 We “see” at different frequencies using different detectors (radios,
  IR cameras, eyes, etc)

Lecture 16, Electromagnetic waves   Phys 100, How Things Work

				
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