# PowerPoint Presentation - Microwaves and Radios

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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

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

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
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

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

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/

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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 views: 35 posted: 3/12/2012 language: English pages: 27