Download Understanding Electromagnetic Waves: From Microwaves to Lasers in Physics 100 and more Study notes Classical Physics in PDF only on Docsity! 1 Phys 100, How Things WorkLecture 16, Electromagnetic waves 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? Phys 100, How Things WorkLecture 16, Electromagnetic waves 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) Phys 100, How Things WorkLecture 16, Electromagnetic waves 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) Phys 100, How Things WorkLecture 16, Electromagnetic waves 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 7-13 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 (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) Phys 100, How Things WorkLecture 16, Electromagnetic waves 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 Phys 100, How Things WorkLecture 16, Electromagnetic waves 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 E 2 Phys 100, How Things WorkLecture 16, Electromagnetic waves 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 Phys 100, How Things WorkLecture 16, Electromagnetic waves 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 Phys 100, How Things WorkLecture 16, Electromagnetic waves 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. Phys 100, How Things WorkLecture 16, Electromagnetic waves 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!) Phys 100, How Things WorkLecture 16, Electromagnetic waves All Waves Follow Similar Rules Reflection Refraction Diffraction Interference Standing or traveling Phys 100, How Things WorkLecture 16, Electromagnetic waves 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
