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AUGUSTANA PH 202 - Electromagnetic Radiation

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Electromagnetic RadiationWhat is Light?The Electromagnetic SpectrumEM SpectrumThe EM SpectrumAtmospheric TransmissionSensitivity of Your EyeIntensity of LightInverse Square LawRadiation PressureComet Hale-BoppComet TailsMomentum TransferLight PressureExample: Light SailThe EM WaveWave EquationsTraveling EM WaveKey ConstantsPoynting VectorNext TimeElectromagnetic RadiationPhysics 202Professor Lee CarknerLecture 20What is Light? A light wave has no medium A light particle is called a photonThe speed of light in a vacuum is a constant, called cc = 3 X 108 m/sAs for all waves, f = v = cThe Electromagnetic SpectrumWe often think of light as being visible light Visible light is just the portion from 400-700 nanometers (nm) Radio waves, microwaves, gamma rays etc. are all forms of electromagnetic radiation with different wavelengthsWe will use the terms “light”, “photons” and “electromagnetic (EM) radiation or waves” interchangeablyEM SpectrumThe EM SpectrumRadio> 1 meter Millimeter (microwave)1 m - 1 mm Infrared1 mm - 700 nm Visible700-400 nm Ultraviolet400 nm - 100 A X-ray100 A - 0.01 A Gamma Ray< 0.01 Ahard to produce and dangerousAtmospheric TransmissionGamma+X-rayInfraredO2, N2AbsorptionH2O, CO2AbsorptionSensitivity of Your EyeIntensity of Light If a light source has a power Ps (in J/s), then the intensity at any point is:I = Ps / 4r2This can also be written:F = L / 4d2Where F is the flux (J/s/m2) and L is the luminosity (J/s) Light (like sound) falls off with an inverse square lawInverse Square LawRadiation Pressure If someone shines a flashlight on you, the light is trying to push you away EM pressure is due to the fact that light has momentum which can be transmitted to an object through absorption or reflectionComet Hale-BoppComet TailsMomentum Transfer p = U/cWhere p is the momentum change and U is the energy change For reflection the momentum change is twice as much:p = 2U/cLight PressureFrom Newton’s second law The amount of energy delivered in time t is:where I is the intensity and A is the areaSince pressure (pr) is force per unit area the pressure becomes:pr = I/c (total absorption)pr = 2I /c (total reflection)Example: Light Sail A light sail is a very large, very thin, very reflective piece of fabric to which a spacecraft is attached Can also use a laser Do need very large sailsThe EM WaveLets consider light as a wave What is oscillating?An EM wave consists of an electric field wave (E) and a magnetic field wave (B) traveling together An EM wave is transverse (like string waves)The field waves are sinusoidal and in phaseWave EquationsWe can generalize the waves as:E = Em sin (kx -t)B = Bm sin (kx -t)Nothing is actually moving A moving E field induces a B field The two fields continuously create each otherThe speed of the wave is related to the fields:Traveling EM WaveKey ConstantsTwo important constants in E and M are the permittivity constant 0 and the permeability constant 0 0 = 8.85 X 10-12 F/mIn farads per meterMeasure of how electric fields propagate through space 0 = 1.26 X 10-6 H/mIn henrys per meter The wave speed depends on these constants:c = 1/(0 0)½Poynting Vector flux = W/m2 = J/s/m2Flux for an EM wave can be given by the Poynting vector:S = (1/0) EB = S = (1/c 0) E I = (1/c 0) Erms2Where Erms is the root-mean-square value of the electric fieldNext TimeRead:


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