L18 : Refractive index - Ray Optics, Physics, Class 12 Video Lecture

hello friends this video on ray optics
part 18 is brought to you by exam feel
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sure that you have watched all the
videos to the part 17 before going ahead
with part 18 now let us introduce a term
called a refractive index so what does
this refractive index can you get
something from the term because it is
always good to guess things from atoms
so that you do not forget what it means
when I talk about when I say a
refractive index the term refractive
itself tells that it is something
related to refraction
what does index dense index is something
which is and when you hear the word
index you mean you feel it is something
which tells about measure of something
it measures something right so it is
basically a measure of bending of light
during refraction because we told that
during refraction the light will Bend
when it enters a different medium so how
much the light will Bend that is
governed by the refractive index of the
medium so refractive index is basically
a property of the medium so depending
upon the refractive index of a medium
the bending of light will happen right
so it is and we can write it in terms of
express expression we write it up we
write it as the ratio of sine of angle
of incidence to the sine of angle of
refraction did you remember something
just in the previous slide I told you
about Snell's law which told that sine
of angle of incidence by sine of angle
of refraction is equal to constant for a
specific pair of media and for a
specific color right so this constant is
nothing but refractive index so
refractive index is often denoted by n
or it is sometimes denoted by mu so
depending upon I mean sometimes you give
I mean make me at many places you see it
is denoted by mu and in many places you
see that it is denoted by n so this
refracted
axis always with respect to some other
media later suppose if I have to medium
one and two this is the boundary between
the two medium and this is the normal to
the boundary let us suppose this is a
ray of light which is incident on the
boundary after refraction it bends in
this way so this is the angle of
refraction right so when I talk about
this refractive index it is always
denoted as n - 1 that means the
refractive index of medium 2 with
respect to medium 1 right so when I say
n - 1 that means refractive index of
medium 2 with respect to the refractive
index of medium 1 so refractive index is
always with respect to something else
right so if I say refractive into one is
refractive index of 2 with respect to 1
we can write it as 1 divided by N 1 2
that means refractive index of 1 with
respect to 2 because refractive index of
1 with respect to 2 would be N 1 by n 2
right so this is refractive index of a
medium you will get better understanding
of refractive index as we go ahead and
we talk about the optical density of a
medium because they are refractive index
plays a very important role
now this reflective index is also
defined as the ratio of the velocity of
light of a given wavelength in empty
space to its velocity in a medium so
what is the velocity of light light in
empty space in empty space it is denoted
by C that is 3 into 10 to the power 8
meter per second and in a medium let us
say it is V so we can say that any
attractive index is equal to C by V so
this is also an our way of defining
refractive index of a medium because in
different medium light travels with
different velocities right so the ratio
of the velocity of light in vacuum
divided by the velocity in any other
is defined as the refractive index so
you can see that a refractive index of a
medium decides with what the velocity
light will travel in a particular medium
so therefore it in turn it decides how
much light will Bend when it enters a
particular medium so refractive index
depends upon the nature of the medium
that means refractive index for water
will be different than refractive index
of glass than refractive index of air so
different medium will have a different
value of refractive index it is
independent of the angle of independent
and gle of incidence because as I
mentioned it is a property of the medium
it has nothing to do with the angle of
incidence of the light now we will talk
about the optical density of a medium
like we know that we are quite familiar
with the term mass density what is mass
density mass density is defined as mass
per unit volume in per unit volume how
much mass is there in a particular
object that is defined by mass density
similarly when I talk about optical
density of a medium what do I mean I
mean that a medium with greater value of
refractive index so optical density of a
medium is nothing but a property of a
medium which determines how much
refraction will take place that means
everything is related if you see
refractive index refractive index will
determine how much bending of light will
occur bending of light is nothing but
refraction so refractive index decides
how much refraction will take place
right and optical density is again a
property which is decided by the
refractive index so optical density is
something which will again decide how
much the refraction will take place so
optical density is directly related to
the refractive index so we have two
types of medium optically denser medium
and optically rarer medium so what is an
optically denser medium we call a media
as optically then said if its refractive
index is more with respect to the other
medium I mean this is always up since it
is denser rarer so it is a comparative
degree that means if I say that medium
one is come is optically denser medium
that means it is optically denser from
than some other medium right so if I say
medium one is optically denser than
medium two that means refractive index
of 1 is greater than refractive index of
two so let us look at this example as I
mentioned earlier we see that for an
optically denser medium the refractive
index is higher and optically rarer
medium the refractive index is lower for
example let us suppose we have two
medium one and two ok so let us suppose
if I say that n - 1 what is n - 1 it is
nothing but refractive index of 2 with
respect to 1 so this is written as n 2
by N 1 so it is refractive index of 2
with respect to 1 if we say that n - 1
is greater than 1 what does this mean
which is a rarer medium and which is a
denser medium if n - 1 is greater than 1
that means n 2 by N 1 is greater than 1
which implies that N 2 is greater than N
1 that means refractive index of 2 is
greater than refractive index of 1 that
means 2 is optically denser and 1 is
optically rarer right similarly if we
say that n - 1 is less than 1 that would
mean that n 2 is less than N 1 that
means refractive index of 2 is less than
refractive index of 1 so in that case 2
will become an optically rarer medium
and 1 will become an optically denser
medium right so this is how
effective index plays an important role
in determining which medium is optically
denser than which medium is optically
rare now optical density is not at all
related to mass density of medium
because when we talk about mass density
it is something of itll mass density has
only to do with mass the amount of the
quantity of matter per unit volume but
when I talk about optical density it has
relation only to refractive index and
refractive index of a particular medium
is very specific to that medium so every
medium has a specific refractive index
of its own that is opposed water has a
refractive index of say one point let us
suppose water has a refractive index of
one point five and air has refractive
index 1 so that means water is a denser
medium what it is a optically denser
medium than air now what is the role of
optical density in a refraction of light
that we will see now because depending
upon the optical density of medium the
bending of light takes place so that is
how optical density affects the
refraction of light thank you please
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