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Electromagnetic Radiation Physics 202 Professor Lee Carkner Lecture 20 What 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 c c = 3 X 108 m/s As for all waves, fl = v = c The 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” interchangeably EM Spectrum The EM Spectrum Radio > 1 meter Millimeter (microwave) 1 m - 1 mm Ultraviolet 400 nm - 100 A X-ray 100 A - 0.01 A Infrared 1 mm - 700 nm Visible 700-400 nm Gamma Ray < 0.01 A hard to produce and dangerous Atmospheric Transmission O2, N2 Absorption Gamma + X-ray H2O, CO2 Absorption Infrared Sensitivity of Your Eye Intensity of Light If a light source has a power Ps (in J/s), then the intensity at any point is: I = Ps / 4pr2 This can also be written: F = L / 4pd2 Where F is the flux (J/s/m2) and L is the luminosity (J/s) Light (like sound) falls off with an inverse square law Inverse Square Law Radiation 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 reflection Comet HaleBopp Comet Tails Momentum Transfer Dp = DU/c Where Dp is the momentum change and DU is the energy change For reflection the momentum change is twice as much: Dp = 2DU/c Light Pressure From Newton’s second law The amount of energy delivered in time Dt 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 sails The 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 phase Wave Equations We can generalize the waves as: E = Em sin (kx -wt) B = Bm sin (kx -wt) 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 Wave Key Constants Two important constants in E and M are the permittivity constant e0 and the permeability constant m0 e0 = 8.85 X 10-12 F/m In farads per meter Measure of how electric fields propagate through space m0 = 1.26 X 10-6 H/m In henrys per meter The wave speed depends on these constants: c = 1/(m0 e0)½ Poynting Vector flux = W/m2 = J/s/m2 Flux for an EM wave can be given by the Poynting vector: S = (1/m0) EB = S = (1/c m0) E I = (1/c m0) Erms2 Where Erms is the root-mean-square value of the electric field Next Time Read: 33.8-33.10