##
This **preview** shows page *1-2-20-21*
out of 21 **pages**.

*View Full Document*

End of preview. Want to read all 21 pages?

Upload your study docs or become a GradeBuddy member to access this document.

View Full Document**Unformatted text preview:**

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 photonThe speed of light in a vacuum is a constant, called cc = 3 X 108 m/sAs for all waves, f = v = cThe 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 Ahard 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 / 4r2This can also be written:F = L / 4d2Where 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/cWhere p is the momentum change and U is the energy change For reflection the momentum change is twice as much:p = 2U/cLight PressureFrom Newton’s second law The amount of energy delivered in time t is:where I is the intensity and A is the areaSince 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 WaveLets 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 EquationsWe 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 otherThe speed of the wave is related to the fields:Traveling 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/mIn farads per meterMeasure of how electric fields propagate through space 0 = 1.26 X 10-6 H/mIn henrys per meter The wave speed depends on these constants:c = 1/(0 0)½Poynting Vector flux = W/m2 = J/s/m2Flux for an EM wave can be given by the Poynting vector:S = (1/0) EB = S = (1/c 0) E I = (1/c 0) Erms2Where Erms is the root-mean-square value of the electric fieldNext TimeRead:

View Full Document