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 SpectrumWe 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 wavelengthsWe will use the terms “light”, “photons” and “electromagnetic (EM) radiation or waves” interchangeablyEM SpectrumThe EM SpectrumRadio> 1 meter Millimeter (microwave)1 m - 1 mm Infrared1 mm - 700 nm Visible700-400 nm Ultraviolet400 nm - 100 A X-ray100 A - 0.01 A Gamma Ray< 0.01 AAtmospheric 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 / 4r2This 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 = 2U/cLight Pressure F = p/t U = I A twhere I is the intensity and A is the area pr = I/c (total absorption)pr = 2I /c (total reflection)Example: Light SailRadiation 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 fuelThe EM WaveLets 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 EquationsWe can generalize the waves as: Nothing is actually moving A moving E field induces a B fieldA moving B field induces an E field The speed of the wave is related to the fields:c = E/BTraveling EM WaveKey ConstantsTwo 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)
View Full Document