All light is made of electromagnetic waves. This means that like any wave, there is
something repeatedly sloshing back and forth with light. A water wave is crests and troughs on the
water going up and down over and over as they travel across the surface. Light waves have some resemblance in that
with light, it is electric and magnetic fields which are periodically wobbling
back and forth.
Light also happens to come in little individual packets of
energy known as photons. Each photon has
only a single portion of an electric and magnetic field which vibrate
perpendicular to each other. This means
that if the electric field is vibrating up and down as it is coming towards
you, then the magnetic field is vibrating left and right. Most natural light will have this orientation
of each photon randomly directed (although the magnetic field of any photon
will always be at a right angle to the electric field).
Light is called unpolarized when the fields are randomly
organized in different directions in this way.
Sunlight, light bulbs, fire and most light sources emit unpolarized light. In this sense, most light waves are more like
the smaller waves on the ocean or those in a swimming pool with lots of people
in it. There is not a simple circular
pattern coming from an isolated source but rather a hodgepodge of lots of these
circular patterns coming from many sources all at once. With light, the electric field of any photon
could be at any angle as it travels. The
photon is just as likely to have its electric field going up down as it is to
have it going left and right or at any angle for that matter.
Light can be polarized using multiple methods although one
very common way this occurs is through reflection. Due to the interactions of the electric field
with the dielectric surface of a mirror or other reflector, reflected waves
have higher polarization than the source terms to the reflection. There is even a special angle that will cause
all reflected light to be polarized.
Most sunglasses today are polarized meaning they only pass
light of a particular polarization angle.
Because LED watches only generate polarized light, if you look at an LED
watch through polarized sunglasses and then rotate the watch or the glasses,
you will see the watch indications disappear as the polarization angle of the
lens passes through the zero transmission angle.
A Light-wave Amplification by Stimulated Emission of Radiation
(LASER) uses these principles in its creation.
By taking a chamber similar to a fluorescent light pumped full of
electrical energy with carefully tuned mirrors spaced in the chamber, a LASER
can be created. If the mirrors are
spaced to be an integer number of wavelengths of the natural photons emitted by
fluorescence of the gas, polarized light can be reinforced by successive
generations of photons created by the gas.
Like any fluorescent light, the gas is energized by passing
an alternating electric field across it exciting electrons in the atoms to
multiple higher energy levels. When the
electrons relax down to lower energy levels, if a pair of the energy levels has
a difference equal to the energy of a visible photon, then a visible photon
will be emitted whenever an electron falls down between these steps. If the mirrors on opposite sides of the
chamber are spaced a whole number of wavelengths apart, they can be made to
reflect only polarized photons of this wavelength. It is when these polarized photons interact
with an excited atom that they stimulate it to only give off an identical
photon, polarized and all. Basically ,
the electric field of the incoming polarized photon causes an excited electron
to move just as it is moving, kind of like an antennae. By having the polarized photons bounce back
and forth through the gas repetitively, the number of these photons is greatly
amplified allowing a coherent beam of the photons to be generated which can
then pass through a partial mirror allowing some to exit as the familiar LASER
beam.
The placement of the mirrors requires fairly perfect
alignment perpendicular to the beam yet both mirrors must be precisely
parallel. This results in the LASER being
created in an extremely straight line making these very useful for industrial
applications of projecting linear markers such as for cutting and welding.
When a laser is used for cutting or spot melting, it is
through the intense electric field which can be generated by the LASER. When this interacts with the electrons of a
substance which is not transparent, those electrons will be pulled off, then back
and forth from the atoms they were previously attached to leaving the atoms
without bonding electrons and effectively turning them into a gas. This only works with high powered LASERS where
more energy can be applied to a spot than can be carried away through natural
heat dissipation.
