The real question becomes will those light waves cancel each other out because the timing is off. This can certainly happen—waves can be out of phase (you’ve perhaps seen this at the beach watching waves or ripples overlap and finding sometimes they build to bigger waves, sometimes they cancel each other out). Life seems to do some things that get around this signal loss. But it’s important here to consider that at worst this cancels the strength of the signal. It doesn’t change the direction the light seems to “twist”.
Finally looking at Part C, we can see that taking the mirror of that slinky, whatever direction it may be pointed (D), is what gives us light circularly polarized in the other (anti-clockwise) direction. It is only in this case, with the light twisted the other way and the two beams mixed in equal parts that would we cancel out our polarization.
In the cartoon below in Part A we have circularly polarized light that to the observer seems to turn clockwise as the wave propagates towards them. Even if we change the direction of propogation of that wave it still appears to turn clockwise (B). This is to say it still twists the same way. It is like a “handedness” in three dimensions (one’s right hand flipped over does not make a left hand). When we consider the panels C and D we see light that to the observer the light propagates towards the electric field vector always appears to “twist” anti-clockwise with time.
Polarized light out of phase can certainly cancel out its own signal but in circularly polarized light, the direction of propagation varying would not cancel the signal unless the light was perfectly out of phase and coincident. Importantly this nulling, if imperfect, is only damping the intensity, not the direction or polarization itself. Nature doesn’t tend to produce light perfectly out of phase and coincident; in fact, in some cases molecules even amplify the circularly polarized light signal.
##A signal perfectly in phase with opposite chirality (clockwise to anticlockwise) would cancel the polarization but the signal you’d actually measure the Stokes parameters changing over time.
HOW do these processes polarize light?
Light can be polarized through transmission, reflection, refraction, or scattering.
Polaroid materials polarize light through transmission by only allowing the light with, and E field oscillating in, one plane to pass through it. Unpolarized light passing through a polarizing filter, or polaroid will become polarized as the other directions of oscillation are not permitted. In a polarimeter this effect is used to detect the direction of polarization, that is, when the filter’s permissivity axis and the light’s polarization axis are aligned on ewould receive the strongest signal, when they are orthogonal no light passes through.
On a microscopic level the polaroid filter is comprised of molecules aligned in one direction throughout.
A similar thing occurs when a crystal is used: in some crystals there is one direction of propagation in transmission in which the electric field can oscillate in a given direction (polarization).
On a \microscopic" level the filter is comprised of molecules or crystals (liquid crystals
in the case of our polarimeter) aligned in one direction throughout. The light vibrating
in that same direction is absorbed while the orthogonally aligned light is the component
which passes. The alignment of the molecules effectively in unidirectional strings means
that their degree of freedom is orthogonal to the "string". A polarising filter is not getting
rid of the light that isn't aligned with the transmission axis, but rather is modifying the
polarity of the light. In the case of the ferroelectric liquid crystal modulator used in our
instrument, the voltage applied to the crystals unwinds them, changing their polarising
orientation property (without a current applied they act as a half wave plate).
Light can be polarised by reflecting o a surface such as a liquid body, as we see in the
glint o the ocean. Metallic surfaces do not polarise light as they reflect light with many directions of vibration. Non-metallic surfaces such as water will polarise light parallel to
the surface since the vibrations of the molecules will tend to align this way. This occurs
because the electrons in the water or other material act like dipole radiators which will
not transmit energy along their vibrational axis. The vibrational axis at the surface of
water is aligned to the surface| so the transmitted light is perpendicularly polarised,
but the reflected light is polarised along the parallel axis.
When light is refracted it becomes polarised to some degree. This is related to the
polarisation produced by reflection, as the light that is reflected tends to be polarised
parallel to the surface and the light refracted as it enters the material, such as water,
is polarised perpendicular to the surface. In birefringent material, such as calcite, the
light can be refracted at two different angles and thus with two different polarisations.
In most cases light polarises with an axis normal to the surface; in calcite one beam will
polarise normal to the surface and the other parallel.
Scattering can produce polarisation and is the primary process expected to be behind
the polarised light observations discussed in this thesis. The repeated absorption and
reemission of light can be anisotropic and anisopolar as in regular Mie scattering, or,
depending on the particles scattering the light and the wavelengths undergoing the
process, there can be isotropy and a tendency for a particular angle of polarisation as
well. Rayleigh scattering is an example of this.
Mechanisms for polarisation in the material, which polarise light interacting with it,
rely on the polarising material containing ions or dipoles. The exception to this is when
electronic polarisation takes place (this is due to the charge asymmetry in the electron
The type of polarisation that occurs depends heavily on the type of material; those
comprised of a single element will produce electronic polarisation, and some materials,
such as water, will be capable of producing electronic, ionic, and dipolar polarisation as
the molecules are mixed elements and dipolar. In an atmosphere a combination of these
mechanisms can occur.
In addition, the wavelengths of light affected by these different mechanisms vary. Blue
light, where we see Rayleigh scattering in most atmospheric conditions, is sensitive to
electronic polarisation. Slightly longer wavelengths will begin to show sensitivity to ionic
Phenomena such as rainbows, Rayleigh scattering, effects from magnetic fields and glint
produce signatures of linearly polarised light.
A related manifestation of polarised light that is not discussed here in detail but could
one day be used to detect biosignatures in the form of chiral molecules is circularly
polarised light. Circularly polarised light is a spiralling vector in polarised light, which is
akin to two linearly polarised light vectors out of phase by a quarter of the wavelength
and with an axis of vibration at ninety degrees from one another.