EMWaves
Electric Charge and Field Basics
Fundamental charges are carried by electrons (negative) and protons
(positive).
Charge on the electron is – 1.6 x 10-19 C
Charge on the proton is + 1.6 x 10-19 C
Like sign charges repel, opposite sign charges attract
Ability to exert electric forces on other charges is represented by the electric
field of the charge
Direction of the electric field is that same as the direction of the electric force
that would be exerted on a positive charge when placed in the electric field
Away
from
positive
+
-
Toward
negative
Electric Charge and Field Basics
Fundamental charges are carried by electrons (negative) and protons
(positive).
Charge on the electron is – 1.6 x 10-19 C
Charge on the proton is + 1.6 x 10-19 C
e = 1.6 x 10-19 C
Called the elemental charge
Current and Magnetic Field Basics
A moving charge is called a current
Moving charges have the ability to exert a second type of force on other
moving charges.
This force is called a magnetic force.
Ability to exert magnetic forces on other charges is represented by the
magnetic field of the current
Direction of the magnetic field is determined by a Right Hand Rule (will not be
used in this course)
Electromagnetic Wave Basics
If a charge is made to jiggle (or more appropriately accelerate) it will produce
a varying electric field. If the charge oscillates with a regular period, it will
produce an oscillating electric field, with all the characteristics of a wave.
Charge
v
Electromagnetic Wave Basics
Faraday’s Law: A changing magnetic field will create an electric
field. The direction of the electric field will be perpendicular to the
direction of the magnetic field.
Maxwell’s Proposal: A changing electric field will create a magnetic field.
The direction of the magnetic field will be perpendicular to the direction of
the electric field.
Electromagnetic Wave Basics
James C. Maxwell (1831 – 1879): In 1864 presented a theory which
combined both of these effects into one entity called the electromagnetic
wave.
Since one field was “created” by the other, there was no need for a medium
in Maxwell’s version of electromagnetic theory.
In order to conserve energy, Maxwell calculated that ALL electromagnetic
waves travel at the same wave speed, 3 x 108 m/sec.
c = 3 x 108 m/sec
Electromagnetic Wave Basics
Special Property of EM waves:
* The can be created in and move though a vacuum
* No medium is required for an EM wave to exist.
* EM waves are the only wave phenomena that can exist without a medium
Electromagnetic Wave Basics
Electromagnetic Wave Basics
Electromagnetic wave phenomena
* Gamma Rays
* X-Rays
* Radio Waves
* Microwaves
* Ultraviolet (UV) waves
* Infrared (IR) waves
* Light
Electromagnetic Wave Basics
The Electromagnetic Spectrum
Wavelength in meters
10000
1
.0001
10-8
IR
Radio
Microwave
10-12
X-Ray
UV
Electromagnetic Wave Basics
The Electromagnetic Spectrum
Wavelength in meters
10000
1
.0001
The Visible Window
10-8
10-12
Electromagnetic Wave Basics
The Electromagnetic Spectrum
The range of wavelengths detectible by the
average human eye
Shortest detectible wavelength ≈ 400 mn
Longest detectible wavelength ≈ 700 mn
Electromagnetic Wave Basics
Transparency of the Earth’s Atmosphere
Electromagnetic Wave Basics
Transparency of the Earth’s Atmosphere
Wavelength in meters
10000
Very transparent
1
.0001
10-8
Partially transparent
10-12
Electromagnetic Wave Basics
Transparency of the Earth’s Atmosphere
Wavelength in meters
10000
1
.0001
10-8
IR
Radio
Microwave
Very transparent
X-Ray
UV
Partially transparent
10-12
Electromagnetic Wave Basics
Transparency of the Earth’s Atmosphere
Wavelength in meters
10000
1
.0001
10-8
IR
Radio
Microwave
Very transparent
X-Ray
UV
Partially transparent
10-12
Electromagnetic Wave Basics
Transparency of the Earth’s Atmosphere
Implications:
Earth based astronomy is based upon the receipt and analysis of
electromagnetic waves. Astronomical observations can only be carried
out at wavelengths that can penetrate through the atmosphere to
telescopes.
Doppler
Shift
Astrophysics
and Cosmology
, f
, f
The Wavelengths are the same in all directions
only if the atoms in the gas are not moving.
Doppler
Shift
Astrophysics
and Cosmology
Longer ,
lower f
Shorter ,
higher f
The Wavelengths get “squished” in the direction of motion, and
“stretched” in the opposite direction.
Doppler
Shift
Astrophysics
and Cosmology
In which direction is the bird moving?
Doppler
Shift
Astrophysics
and Cosmology
Longer ,
lower f
Shorter ,
higher f
In the visible spectrum, the color blue is associated with the shorter
wavelength end of the visible window. The squishing of the wave to a
shorter wavelength is called a “blue shift”
Doppler
Shift
Astrophysics
and Cosmology
Longer ,
lower f
Shorter ,
higher f
In the visible spectrum, the color red is associated with the longer
wavelength end of the visible window. The stretching of the wave to a
longer wavelength is called a “red shift”
Doppler
Shift
Astrophysics
and Cosmology
Longer ,
lower f
“Red Shift”
Shorter ,
higher f
“Blue Shift”
Doppler
Shift
Astrophysics
and Cosmology
Longer ,
lower f
Shorter ,
higher f
“Red Shift”
“Blue Shift”
In general
Apparent Wavelength
True Wavelength
=
True Frequency
Apparent Frequency
=
Velocity of Source
1+
Wave Speed
Note: If the source and detector are moving apart, the Velocity of the Source
is POSITIVE. If the source and detector are toward one another, the Velocity of
the Source is NEGATIVE.