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Lecture 16, Electromagnetic waves^ Phys 100, How Things Work
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 = λ f and c = 300,000 km/s = 186,000 mi/s So wavelength is λ = 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 ( λ = 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- 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- FM radio: 88 MHz to 108 MHz ( λ = 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 ( λ = 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 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
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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 I Oscillating charges re-emit the same frequency electric field oscillations
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 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)
