Transcript A Brief History of Planetary Science

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