L29 : Application of Polaroid - Wave Optics, Physics, Class 12 Video Lecture

it hello friends this video on wave
optics part 29 is brought to you by exam
clear calm no more fear from exam please
make sure that you have watched all the
videos to the part 28 before going ahead
with pad 29 let us look at the first
method that is polarization by Polaroids
now before that let me tell you how do
we denote polarized light an unpolarized
light when we talk about polarized light
we talk about unpolarized light we
denote them in this way this represents
cólera direction but by the plane of
vibration parallel to the plane and this
represents partment in the perpendicular
direction so this represents this shows
that there is a plane of vibration
parallel to the plane and there is a
plane of vibration perpendicular so this
is that Delage
this is a way to denote unpolarized
light and how do we denote polarized
light polarized light can be either this
or this that means polarized light can
either be in the perpendicular plane or
it can be the parallel so this is how we
do know polarized light right so with
this basic information let us start to
understand later start understanding
polarization by Polaroid's
so let us consider a Polaroid so what is
a Polaroid Polaroid are polarizing
materials consisting of long chain of
molecules aligned in a particular
direction
so basically polarized some materials
and you can get it in the form of thin
sheets as well as you can get it in the
form of some some quads metal and things
like that so what is it they basically
have molecules where all the molecules
are arranged in such a way that they are
all aligned in a particular direction so
every Polaroid has a past axis so what
does that pass access that pass axis
you can imagine that as a gate of the
Polaroid so it will allow light to pass
only through the fast axis of a Polaroid
it is suppose this is a Polaroid there
will be a thin opening in the middle so
that means this is the door of the
Polaroid you can only pass allow this so
that means if there is a light which is
vertical it can pass through this but if
there is a light along this direction
this will not allow this to cross right
so a polarizer can have horizontal pass
axis it can also have a vertical pass
axis right so these pass axis of the
Polaroid determines how the light will
pass through it so what happens when an
unpolarized light is passed through a
polarizer cooler at Polaroid
it gets polarized how here if you look
at this this is unpolarized light
because it has plane of vibration in all
possible directions right so if you look
at it it has end if it is vibrating in
all different planes at different
instant of time now as soon as it passes
through this Polaroid let us suppose if
this is the slit of the Polaroid it will
only allow those vibration which is
along this direction so that means the
remaining vibration will not be allowed
to pass through the Polaroid so only
these vibrations will pass and as a
result the light will be polarized right
are you understanding the concept if
there is an opening along this direction
it will only allow the light which is
oscillating in this direction it will
not allow light which is oscillating in
this fashion or this fashion to pass
through this opening the light which is
oscillating along this direction can
only pass through this Polaroid thank as
a result the light becomes the
unpolarized light becomes polarized so
there was a series of experiment which
was performed using colloids to study
the effects of polarization so the first
one was the first part of the experiment
was polarization by a sting
in Poland what happened was that the
same thing as I mentioned before one
Polaroid was kept and it was found that
the light which was emitted which came
after coming crossing the Polaroid was a
polarized light and it was observed that
the intensity is almost half that is
because initially there were so many
light waves right now almost half of
them have been cut off so only the
remaining half is allowed to enter and
as a result the intensity is almost hard
because half of them has been cut off
they are not allowed to enter so that
was one thing that was observed and the
next observation was that rotation of
the Polaroid has no effect on the
intensity of transmitted polarize flag
that's because even if you rotate the
polarity suppose initially the accesses
the pass axis is along this direction if
you rotate it it will be somewhat like
this if you rotate it further it will be
somewhat like this but at any
orientation at any of these orientations
it will allow only half of the light to
pass through it that's because in the
light is in this unpolarized light you
have equal amount of light in all
possible directions so whichever
direction you rotate the Polaroid the
same amount of light will be allowed to
pass through let us suppose you have you
take this example to understand this
concept let us suppose you have three
let us suppose you have total thirty
chocolates or if suppose you have got
total thirty pens right out of which you
have ten Reynolds you have got ten
pencils and you have got n Margot's you
have 10 pens 10 pencils and 10 markers
now let us suppose there is a box which
will tell you what it will it will
accept there is a box which will accept
either pen or pencil or marker it will
not accept
any two varieties right so if the box
says that it will allow pens in that is
you pour all the 10 pens so what is the
result in so how many objects are there
inside the box then if the box is it
will accept only pencils you will pour
all the 10 pencils so how many objects
are there inside the box then if it says
it will accept only marker you will put
all the markers so how many objects are
there then
so that means irrespective of whether
you are putting pen pencil or marker the
number of objects inside this box is all
instant that is because each of these
objects are present in equal number all
of them are present in 10 numbers like
similarly here the different load mean
there are many different orientation but
all of them are presenting equal amount
so whichever may be the direction of the
Polaroid in that particular direction
same amount of light is there so every
time as the amount of light which comes
out of the polarizer or the intensity of
life which comes out of the Polaroid
remains the same sometimes it will allow
the vertically polarized light to pass
through sometimes it will allow the pan
the perpendicular polarized light to
pass through so direction might change
but the intensity of the light will
remain the same so in the second part of
the experiment to polaric Polaroids when
used so earlier there was only one
Polaroid now what was done another
polarized was introduced in between so
this is my new Polaroid BB
this was my previous polaric so
previously what was happening when the
light was coming plane polarized light
was coming out and when the when p1 was
rotated there was no effect right that
was what was observed in the first
experiment in the second experiment it
was observed that when another Polaroid
p2 was placed in between the unpolarized
light and in previous polarizer it was
seen that the intensity of the light is
almost half after keeping
let us quite obvious right because
polarized light is coming it is passing
through this polarizer so this polarizer
will again allow only anyone claim of
vibration so as I design you get a
linearly polarized light and at this
point of time itself the intensity is
half right now what happens when this
polarized light now enters the second
polarizer now there are different
possibilities now it depends on the
orientation of the axis of this
polarizer now if this polarizer p1 is
having its fast axis along the same
direction as peaking then what will
happen it will allow the light to come
out of it
so here the intensity was half and here
also the intensity will remain the same
suppose here the intensity was I then
here also the intensity will be I
because both of them are their axes are
along the same plane so whatever waves
tup to allowed to pass the same waves p1
will also allow to pass so as a result a
intensity will remain the same here but
in case if the two axes are not oriented
in the same direction that is if I
rotate Polaroid p1 it will have effect
on intensity of the transmitted light
now suppose if I rotate p1 in such a way
that its axis is perpendicular to the
axis of people that means let us suppose
if axis of p1 is like this and axis of
PEEP the access of p2 is like this and
p1 is like this if axis of p2 is like
this it will allow the waves which are
oscillating like this now when this
waves will reach these polarizer this
will not allow because the waves are
oscillating like this and you have the
opening like this so that means this
will not allow any way and as a result
the net intensity will be zero so this
is the scenario in this case the net
intensity is high but in this case the
net intensity is zero it will not
transmit any light so here axis of p1 is
value delta t/2 in this case and in this
case axis of p1 is perpendicular to the
axis of people so this
means that as we keep rotating p1 the
intensity of the transmitted light keeps
changing and the intensity is minimum
when p1 is perpendicular to people that
is transmitted intensity is zero then
pass access of p1 is perpendicular to p2
and it is maximum when pass axis of p1
is parallel to p2 so this was a very
important result
I mean this experiment one in experiment
2 together helped us to study the
polarization by Polaroids and using
these experiments it was concluded that
intensity of the polarized light varies
with the angle between the past axis of
the two Polaroids right and that is
absolutely correct
intensive here in the previous slide we
have just discussed about the two
extreme cases one is when the angle
between the two pass axis is zero and
the other is the angle between the two
powers axis is 90 degrees but even in
between them the intensity will keep
varying right so that means it was
something like this intensity is zero
when theta is 90 degree and intensity is
maximum when theta is equal to zero
degree right what happens so now the
question is we are talking about two
extreme conditions when theta is zero
and theta is 90 it shows that as the
angle theta changes from zero to 90 as
the angle increases from zero to 90 the
intensity keeps decreasing so what
happens when the angle between the pass
axis is some angle theta between zero
and 90 degree so how does it vary with
theta so for that came up this famous
law called the malleus law Annihilus
gave a law because he experimental he
found that the intensity values as
cosine square of theta where theta is
the angle between the pole between the
pass axis of the two Polaroids where
what is I naught I naught was the
intensity of polarized light after
passing through p1 and I was the new
intensity after rotation
before rotation the intensity was I not
and after rotation the intensity is and
so here I not and I both are the
intensity of the final transmitted light
that means let us suppose this was
unpolarized light right it was passed
through one polaroid then it became
polarized then it was again passed to
another polaroid so the final intensity
which we get this final intensity before
rotation was I not and after it was
rotated is alpha so then I is related to
I not as I is equal to I naught cos
square theta so this is how intensity
values with the orientation of the whole
era
so this was one method by which we can
polarize like so in this way we can
polarize the unpolarized light so but
why thank you please visit exam few calm
to watch free educational videos try
through your online tests get the best
quality study materials study from the
best tutors and mentors and much more
thank you once again