electromagnetic radiation physics 202 professor lee carkner lecture 20
Post on 18-Dec-2015
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![Page 1: Electromagnetic Radiation Physics 202 Professor Lee Carkner Lecture 20](https://reader030.vdocument.in/reader030/viewer/2022032800/56649d235503460f949f9b4d/html5/thumbnails/1.jpg)
Electromagnetic Radiation
Physics 202Professor Lee
CarknerLecture 20
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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 cc = 3 X 108 m/s
As for all waves, f = v = c
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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
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EM Spectrum
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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 hard to produce and
dangerous
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Atmospheric Transmission
Gamma+
X-ray
Infrared
O2, N2
AbsorptionH2O, CO2
Absorption
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Sensitivity of Your Eye
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Intensity of Light
If a light source has a power Ps (in J/s), then the intensity at any point is:
I = Ps / 4r2
This can also be written:F = L / 4d2
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
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Inverse Square Law
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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
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Comet Hale-Bopp
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Comet Tails
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Momentum Transfer
p = U/c Where p is the momentum change and
U is the energy change
For reflection the momentum change is twice as much:
p = 2U/c
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Light Pressure From Newton’s second law
The amount of energy delivered in time t
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)
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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
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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
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Wave Equations We 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 other The speed of the wave is related to the fields:
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Traveling EM Wave
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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
In farads per meter Measure of how electric fields propagate through space
0 = 1.26 X 10-6 H/m
In henrys per meter
The wave speed depends on these constants:c = 1/(0 0)½
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Poynting Vector
flux = W/m2 = J/s/m2
Flux for an EM wave can be given by the Poynting vector:
S = (1/0) EB = S = (1/c 0) E
I = (1/c 0) Erms2
Where Erms is the root-mean-square value of the electric field
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