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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 VectorIntensityElectromagnetic RadiationPhysics 202Professor Lee CarknerLecture 20What is Light? A light wave has no medium A light particle is called a photon c = 3 X 108 m/sThe 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Atmospheric 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: 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 The above equation is for absorption p = 2U/cLight Pressure F = p/t U = I A twhere I is the intensity and A is the area pr = I/c (total absorption)pr = 2I /c (total reflection)Example: Light SailRadiation pressure can be used to power a spacecraft The sail can gather light from a star to propel the spacecraft Light sail powered craft need no engines or fuelThe EM WaveLets consider light as a wave  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: Nothing is actually moving A moving E field induces a B fieldA moving B field induces an E field The speed of the wave is related to the fields:c = E/BTraveling 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 Measure of how electric fields propagate through space 0 = 1.26 X 10-6 H/m Measure of how magnetic fields propagate through space c = 1/(0 0)½Poynting Vector The amount of energy delivered per unit area per unit time is given as flux: Flux for an EM wave can be given by the Poynting vector: However, E and B are related by E/B = c so we can rewrite S as:S = (1/c 0) E2Intensity We generally are interested in the time averaged value of S, known as the intensityI = (1/c 0)


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