Refraction ; Refractive index - Light Reflection & Refraction, Science, Class 10 Video Lecture - Class 10 | Best Video for Class 10

hello friends this video on light
reflection and diffraction patterning is
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move here from example please make sure
that you have watched all the video
stand back 12 before going ahead with
part 13 so refraction of light is
another beautiful phenomenon of light we
will see what happens in case of a
refraction of light in the next few
slides so let us now look at what does
the refraction of light till now we
spoke about the reflection of light
which meant the bending or the bouncing
back of light when it strike a boundary
between two media so what is refraction
of it is the bending of light when it
passes from one medium to another so
that means there is no bouncing back now
it says that light will pass from one
medium to another but the way of light
instead of going straight it will bend a
little so that is known as refraction of
light there are several examples around
us where we see this phenomenon of
refraction for example the first picture
depicts a pencil dipped in a glass of
water you can try it on your own take a
glass of water put a pencil inside that
water what do you see that the pencil
appears little bend as you can see here
also right why why does this happen the
pencil is not at all Bend right it is a
straight pencil this due to the
diffraction of light because here light
rays are entering from one medium to
another one medium is air the other
medium is water so when it enters from
one medium to another light rays Bend
now due to this bending of the light
rays we the object appears little
different than one what it is actually
similarly if you look at this aquarium
you see that there is a fish which is
visible in a different way I mean you
see the actual position and the apparent
position I mean the position where the
fish actually is and the position where
you are viewing the fish are different
that is again due to the federal
no refraction because here also two
medium are involved here and water
similarly the rainbow which you see in
the sky is also a result of refraction
of light mostly you would have seen that
a rainbow is observed after after the
rain I mean after any rainfall when one
part of the sky is little sunny whereas
the other part is little cloudy you
generally see a rainbow
so this rainbow is also formed as a
result of diffraction so we will discuss
about the formation of rainbow in detail
and add in one of the later slides so
for now get a stripe of focus on the
phenomenon of refraction what actually
happens in refraction now similar to
reflect reflection there are such there
are two laws of diffraction as well
which has to be universally followed
wherever refraction takes place now in
case of refraction again we have some of
the terminologies that is the incident
ray the grey which falls on the surface
between the two media and this is let us
suppose this is medium one this is
medium two this is my incident ray and
this is mine refracted ray right because
hey if you see the incident ray should
have gone like this but instead of that
it gives slightly bent so this becomes
your refracted ray and this normal is
again the same thing that is
perpendicular to the surface so the
first loss is that incident ray
refracted ray and normal to the
interface at the point of incidence or
lying on the same plane so it is similar
to the law of reflection it says that
the incident ray reflected ray and
normal should lie on the same plane so
in this also see is the same thing
the second law see is that the ratio of
sine of angle of incidence to sine of
angle of refraction is a constant for a
specific light of a specific color and
for a specific pair of medium that means
in this case this is my angle of
incidence that is the angle which is
formed between the incident ray and
bournemouth similarly the angle formed
between the
normal and the refracted ray is known as
angle of refraction so here theta one is
angle of incidence and theta two is
angle of refraction so the second law
see is that sine of angle of incidence
by sine of angle of refraction is equal
to constant now this ratio will remain
constant only for a specific pair of
media for example if this is what air
and this is water so the value of sine
theta 1 by sine theta 2 will remain same
only for this pair now if you keep glass
instead of water the value of this ratio
will change so that means this is
constant only for a specific light of a
specific color that means for a light of
a specific wavelength because of the
color of the light changes the
wavelength also changes so this is true
only for a specific pair of media and
for a specific wavelength of light so
this law is often known as the Snell's
law this is often known as the Snell's
law it is named after the scientist
Snell okay so now we will introduce a
new term that is known as the refractive
index now what is the refractive index
of a medium so what is this refractive
index now as I told that whenever light
enters from one medium to another the
ray of light will Bend now there has to
be some parameter to measure that how
much bending of light will take place
there has to be some factors which will
determine that okay light will Bend this
much when it moves from this medium to
this medium it will move that much when
it moves from this medium to this middle
so there has to be a measure of bending
of light so that is why this Vik fractal
index was introduced it is the measure
of bending of light during refraction
that means how much or to what extent is
the ray of light bending when it enters
from one medium to another medium so how
is it defined it is
find as the ratio of the sine of angle
of incidence to the sine of angle of
refraction just now we saw in Snell's
law that sine of angle of incidence to
the sine of angle of refraction is
constant now this constant is nothing
but this constant is defined as the
refractive index so we define this
constant as n - 1 it is often denoted
like this that means this is refractive
index of medium 2 with respect to medium
1 let us suppose this is medium 1 and
this is medium 2 this is the normal to
the medium this is the ray of light this
is I and this is I so we see that sign I
by cyanide is equal to nd one that is
refractive index of medium 2 with
respect to medium 1 so this is how we
define refractive index of a medium it
is also defined as the ratio of velocity
of light of a given wavelength in empty
space to the velocity in a medium so
that means we are comparing the velocity
of light in a medium with respect to
vacuum so we also define refractive
index of 2 with respect to 1 as C by V
where C is the velocity of light in
vacuum and V is the velocity in some
medium now there is another important
relationship that is refractive index of
medium - with respect to 1 is equal to 1
divided by the fractal index of 1 with
respect to 2 so they are the reciprocal
of each other a refractive index of 2
with respect to 1 is equal to 1 upon
refractive index of 1 with respect to 2
now from these definitions of refractive
index we can say that refractive index
spends on nature of the medium that is
absolutely correct because this velocity
will depend upon the medium if it is
water the velocity of light will be
different if it is glass the velocity of
light will again be different so that
means the value of refractive index
depends or changes with the change in
medium it is independent of angle of
incidence right so nature of the medium
is the only thing that governs the value
of refractive index it does not matter
at what angle of incidence the incident
ray falls on the surface between the two
media right so the only paragraph means
refractive index is a unique for every
medium every medium has a refractive
index of its own now depending on the
value of the refractive index we can
then judge the bending of light how much
the light will Bend refractive index is
the parameter which will tell us about
the optical density of a medium for
example whenever we talk of medium we
categorize them into two types we say
optically rarer medium and optically
denser medium so in this case when we
talk of density we are not talking about
mass density we are talking about the I
mean how dense is that medium to refract
light if the medium has very high
refractive index that means we say that
it is optically denser medium that means
it light enters into that medium the
bending will be more similarly if the
value of the refractive index is very
less we say that the medium is a
optically rarer medium right so value of
refractive index also tells us about the
nature of them medium and the properties
of a specific medium determines how much
refraction will take place in that
medium or how much bending will take
place in that medium so whenever we talk
about the optically denser medium it has
higher refractive index optically rarer
medium always have let low
refractive index let us take this
example later suppose I have two medium
medium 1 and medium 2 now let us suppose
if I say that N 2 1 what is n 2 1
refractive index of medium 2 with
respect to medium 1 if n 2 1 is greater
than 1 what does this mean this means
that refractive index of 2 with respect
to 1 is greater than 1 this means that
refractive index of 2 by refractive
index of 1 is greater than 1 or it's
means refractive index of 2 is greater
than refractive index of 1 this means
that medium 2 is a denser medium and
medium 1 is a rarer medium so here
denser and rarer means optically denser
and optically rarer similarly if we say
that n 2 1 is less than 1
this implies that refractive index of 2
is lesser than refractive index of 1
this means that 2 is a rarer medium and
1 is a denser medium now please make a
note that here optical density is not
related to mass density of the medium in
any ways mass density talks about mass
per unit volume but when I talk about
optical density it means how dense or
how how much more dense or how much less
dense is the medium to bend a ray of
light so if the refractive index is more
it will bend the light more if the
refractive index is less it will bend
the light less right so now you
understand the significance of
refractive index and why it is called
the refractive index at this point
refractive index because it is an index
which defines the extent of refraction
that is how much refraction will take
place
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