- Image 1 Caption:
An animation
showing ray of
electromagnetic radiation (EMR)
propagating
to the right obliquely.
The ray could be part of a continuum
of rays making a beam of finite
geometrical cross section
such as a plane wave.
See Image 3 below for an illustration of
a plane wave.
In fact, for definiteness, let's speak of the beam in the
animation as
a plane wave.
- The small Greek letter
lambda λ is the common
symbol
for wavelength.
- A wavelength is the length of a complete
spatial cycle of the wave motion.
- The oscillations of the
electric field (E-field)
(shown in red)
and the
magnetic field (B-field)
(shown in blue)
have a directions in real space, but they extend in abstract
electric field space
and magnetic field space.
- Image 2 Caption:
An analogue
animation to
Image 1, but
now moving directly to the right.
- The lengths of the
vectors
are really the magnitudes or strengths
of the electric field
and magnetic field.
- The electric field
and magnetic field
are perpendicular to each other and the propagation direction.
The oscillations being perpendicular to the propagation direction makes
EMR a
transverse wave phenomenon
like
waves on a string.
- In EMR, the
electric fields and
magnetic fields
are coupled: i.e., are mutually interacting.
In fact, a time-varying
electric field
creates a time-varying
magnetic field
and vice versa---they create each other to paraphrase
Jack Nicholson (1937--).
And they must do so for
EMR to be self-propagating.
- Image 3 Caption:
A diagram
of a square
segment of linearly polarized
electromagnetic radiation (EMR)
plane wave.
- Ideal plane waves,
extend to infinity.
Realistic ones have a finite extent with complicated edges in general.
However, a small enough portion of any
wavefront
(e.g., of
spherical waves
as in Image 4 below) approximates a
plane wave.
- The directions of the
electric field (E-field)
(shown as red
arrows)
and the magnetic field (H-field)
(shown as green
arrows) are shown in
in real space, but their extent is in, respectively,
abstract electric field space
and abstract magnetic field space.
Note for magnetic fields
either the symbol E or symbol H may be used.
The two symbols do NOT mean quite the same thing, but our purposes they do.
- In the plane wave,
the electric field
and the magnetic field
have a constant direction and magnitude throughout
the any plane
of the plane wave.
These planes
are perpendicular to the
propagation direction given by
black
arrow S.
- The electric field
and the magnetic field
are perpendicular to each other
and are perpendicular to the
propagation direction.
- The plane wave
is a monochromatic
sine wave.
The sine wave behavior
(which represents extent in
abstract electric field space
and abstract magnetic field space)
is shown by the
red and
green bands.
- As for Images 1 and 2,
the small Greek letter
lambda λ is the
symbol
for wavelength.
- The sources of the
EMR beams in the images above
may be oscillating electric charges
at the right, but NOT shown in the images.
- Linear polarization:
- The beams in the images above are
linearly polarized.
This means that the
electric field
and magnetic field are confined
to two perpendicular
planes.
- Real finite beams are made up of many
photons (the particles of
EMR) with all orientations of
the electric field
and magnetic field
perpendicular to the
propagation direction being possible.
- The vectors of the photons do NOT
cancel microscopically---or else
there would be a lot less EMR
than there is.
- Sources that radiate EMR
from their own internal heat energy
naturally radiate
unpolarized light
in which all orientations of
the electric field
and magnetic field perpendicular to the
propagation direction occur with equal weighting.
- EMR
can be linearly polarized
through various processes like
reflection
and transmission through polarizing materials like those
that are used in polarized sunglasses.
A filter that linearly polarizes
is called a polarizer.
- There is also
circular polarization---but that is
the story for another place.
- Image 4 Caption:
A cutaway
animation
of a generic
spherical wave
from a point source.
Far from the point source,
a small part of a wavefront
will approximate a
plane wave.