The phenomena of restricting the vibrations of the light to a
particular plane is called polarization. It is the property of the transverse
wave. A transverse waves can only have vibrations in both the planes. Hence
restricting the vibrations of one plane is possible with the transverse wave.
Sound waves in air cannot be polarized because they’re
longitudinal in nature.
To explain the concept of polarization we shall depend on an
electromagnetic wave theory of light. According to this theory, the varying
electric fields particles generate a varying magnetic field around it and the
direction of wave motion is perpendicular to both of them.
The magnetic field vibrations are no way responsible for any
of the optical properties of light. Electric fields vector is responsible for
all optical properties of light. This electric field vector can be resolved
into components. The horizontal and vertical components exist in the
perpendicular planes. The light that is having both these components is called unpolarized
light or natural light. To polarize the light, we shall restrict either
horizontal vibrations are vertical vibrations. Horizontal vibrations are represented
with arrows and the vertical vibrations are represented with darts.
A unolarised light consists of both the horizontal and
vertical vibrations. When the light is polarized, it will have either only the horizontal
vibrations or only the vertical vibrations.
The intensity and the amplitude of the polarized light is
less than that of the original light. To be more specific intensity of polarized light is half of the original intensity.
Polarization can be done by certain type of crystals like
quartz crystal. This crystal is called as polarizer. To check whether the light
is actually polarized or not, we can use the same crystal and then it is called
as analyzer. Depending on the position of analyzer, the outcome of the light
and its intensity can be identified as shown in the diagram.
Problem and solution
Unpolarised light of intensity that the 32 W per meter Squire
passes through three Polaroid’s such that the transmission axis of the last
polariser is crossed with the first one. If the intensity of the emerging light
is 3 W per meter Squire what is the angle between the axes of the first two?
It is given in the problem that the sum of the two angles is
equal to 90°. We know the equation further resultant intensity and by writing
in this equation and simplifying we can get the answer as shown below.
Problem and solution
A beam of ordinary light is incident and system of four
Polaroid’s which are arranged in succession such that each Polaroid is turned
through 30° respect to the preceding one. What is the percentage of the
incident intensity that emerges out from the system?
We know that when the light is passed through Polaroids its
intensity decreases. If it is passing through only one polariser with the
parallel axis only half of the intensity is going to remain. When there are
good number of the polarisers, to find out the resultant intensity we can use a
formula that is shown below and get the intensity.
Problems and solutions
In the following page attached, three problems are solved
with a detailed solution. A comment can be made at the end of the post for any
kind of further clarifications?
Polarization can be achieved through the process like
reflection, refraction and double refraction. When the light strikes the
boundary that is separating the two media, for a particular angle of incidence
the reflected light is completely polarized. This particular angle of incidence
is called the angle of polarization. In this case the refracted light consists
of a small number of vertical vibrations and hence the refracted light is not
completely polarized.
We can nature the polarization even by refraction. Here a
group of plates are arranged one after the other so that each plate to its own polarization up to some extent. As many number of the required plates are
placed such that the refracted light is having only horizontal vibrations. This
combination is called pile of plates.
We can get the polarization by double refraction also with
the help of so special crystals like a calcite crystal. This crystal has a
specific property of reflecting the light twice and this property is called the
double refraction. It is experimentally observed that both this refracted
light’s are completely polarised.
One of the refracted lights follows all the regular rules of
light and it is called ordinary light Ray. With the ordinary light Ray
refractive index of the medium and the velocity of the light is constant.
The second refracted light Ray doesn’t follow the regular
rules and it is called the extraordinary light Ray.
We can identify the ordinary and extraordinary light Ray
quite easily. When the calcite crystal is placed in on a point object it makes
to point images. With the rotation of the crystal one image reminds in the
state of rest and the image is as ordinary. With the rotation of the crystal
other image starts rotating and it is called the extraordinary.
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