Polarization by Scattering and Reflection Video Lecture | Physics for JEE Main & Advanced

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with part 31 now we will look at
polarization by reflection and
refraction so how reflection and
refraction polarized light let us
suppose this is the boundary this is
mine incidentally this is the Nordmann
this is my reflected wave and this is my
refracted ray okay so here on the
incident ray we have unpolarized light
so how do we denote unpolarized light
and polarized light width in is denoted
in this thing where the dot represents
the plane perpendicular to the direction
of incident ray and this line indicates
the direction where dot represents the
direction along the plane of the paper
and this line represents this represents
parallel to the plane of the paper and
this represents perpendicular to the
plane of the paper right so what happens
when this ray is incident on this
boundary some of the Rays leave
reflected and the remaining is refracted
so this is my angle of incidence this is
my angle of refraction right now it was
analyzed and found that most of the
light in the reflected ray they are
polarized parallel to the plane with a
very few unpolarized component whereas
most of the light along the refracted
ray
is unpolarized with one or two polarized
component so that means
both they reflected ray and the
refracted ray both of them are partially
polarized the refracted light wave is
more unpolarized and the reflected light
wave is more polarized so this was found
now it was also observed that as the
angle of incidence was varied that means
as the Ray was made to fall incident
from this set what happened more and
more unpolarized light wave hit entering
inside the surface it was something like
this it is suppose you have two
components one vertical component and
one parallel component now if you
incline it more if you incline it more
in this way more and more will enter
inside it is something like you if you
have a sharp object and if you have the
ground
if you fall if you try to fall that if
you try to throw that sharp object
towards the ground it will go inside the
ground okay but if the object is blank
it will not go inside the ground
incident it will bounce back so similar
thing is happening here these are these
components are the perpendicular
component so they are the sharp
components so when you project it in
such a way that most of them go inside
that is why most of them most of them
colorized light could get into the
refracted width and very few of the
parallel components bounces back and
form the reflected ray now if you change
this angle of incidence I mean if this
angle of incidence had been from this
point then what happened very few of the
components entered inside and also this
became more unpolarized similarly it was
found that as the angle of incidence
where there was a particular value of
angle of incidence at which this
reflected ray was maximum polarized so
below that value of incidence and above
that value of incidence both the waves
were partially polarized so the bottom
line is here is that when an unpolarized
light falls on the boundary both the
reflected wave and the refracted waves
are partially polarized the reflected
wave is more polarized and the reflect
different
is more unpolarized here we'll talk
about the Brewster's law which talks
about the variation in polarization with
the angle of incidence which I was
talking about in the previous slide at
any particular angle of incidence the
reflected ray is completely polarized
there is a particular value for which
this angle of incidence is called
registers angle for which the reflected
ray is completely polarized and at such
situation the angle between the
reflected and refracted ray is 90 degree
that means this angle is 90 degree under
such situation right now if you change
this angle I mean if this angle is
changed to some value here again this
will become little unpolarized if you
change this value and tilt it a little
more then what will happen more and more
unpolarized will enter inside the inside
this medium right so that means there is
this value of angle of incidence for
which the reflected wave is completely
polarized other than this value
everywhere the reflected ray and the
refracted ray both are partially
polarized so this we can so we can find
out an expression for the Brewster's
angle this angle of incidence is known
as boosters angle after this which is
named after this law so act I is equal
to IB what happens when angle of
incidence is equal to the Brewster's
angle that time this total angle is
equal to 90 degree right now from
Snell's law we know that sine I by sine
R is equal to MU what is sine I sine I
is nothing but sine IB because angle of
incidence is brinster Sanga what is
angle R R is nothing but this angle of
refraction so what is this angle of
refraction that is nothing but that will
be nothing but PI by 2
- dream strangle why is it so that's
because the angle it is told that the
angle between reflected and refracted
ray is 90 degree that means this angle
is 90 degree so what is this angle if
you look at it this angle I be angle of
incidence is always equal to angle of
reflection so that means this is IB this
is IB this total is 90 degree so we have
to find out this angle so if this is IB
and this is 90 so this is angle IB
because angle of incidence is equal to
angle of reflection this total and then
is 90 so that means what is this angle
this angle is equal to 90 degree minus
IB again this total angle is equal to 90
degree so that means what is this angle
this angle is IB so this is IB and this
is angle of refraction therefore we can
write IB plus R is equal to 90 degree so
we can say R is equal to 90 degree minus
ID so that is what we have written here
this is equal to MU so sine 90 minus
theta is nothing but cos theta so we can
write it as cos I D this is equal to MU
therefore we can write tan IB is equal
to MU so this is the expression for
rooster angle
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