# Electromagnetic Radiation Physics 202 Professor Lee Carkner Lecture 20.

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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  c = 3 X 10 8 m/s

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   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 Transmission Gamma + X-ray Infrared O 2, N 2 Absorption H 2 O, CO 2 Absorption

Intensity of Light   If a light source has a power P s (in J/s), then the intensity at any point is: I = P s / 4  r 2  This can also be written:  Where F is the flux (J/s/m 2 ) 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 Hale- Bopp

Comet Tails

Momentum Transfer   p =  U/c   The above equation is for absorption   p = 2  U/c

Light Pressure  F =  p/  t   U = I A  t  where I is the intensity and A is the area  p r = I/c (total absorption) p r = 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 phase

Wave 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/B

Traveling EM Wave

Key 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 ) ½

Poynting 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 ) E 2

Intensity   We generally are interested in the time averaged value of S, known as the intensity I = (1/c  0 ) E rms 2 