Rainbow Formation - Human Eye, CBSE Class 10 Physics Video Lecture

hello friends this video on human eye
and the colorful world part six is
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part six now let us look at an
interesting example of refraction that
is rainbow so it is an interesting
example of dispersion refraction and
reflection of sunlight by spherical
grain water droplets so it is an example
of all the three phenomenon which we
have studied so far
they studied reflection listed in
refraction and just now we studied about
this portion as well so rainbow is an
example of all these three phenomenon so
let us try to see how exactly how and
when is a rainbow form when do you
actually see a rainbow
if you observe you will generally see a
rainbow after rain then later suppose
you had a heavy rain when the rain
almost stops such that one part of the
sky is sunny whereas the other part of
the sky is raining now if you stand
keeping your back towards the Sun you
will be able to see the rainbow that is
the observation now why that happens
that I will explain you in the next two
slides so did you understand when
exactly you see a rainbow you you will
observe it for yourself that when the
rain stops and the weather research that
some part of the sky is sunny you can
see the Sun there and some part of the
Sun some part of the sky is still cloudy
now if you stand in such a way that your
back is towards the Sun in that case you
will be able to see the rainbow in front
of you right okay so in this picture
also you can see rainbow so if you see
this part of the sky is cloudy whereas
this part of the sky is little sunny so
if you stand somewhere here you will be
able to see this rain now there are two
types of rainbow one is called a primary
rainbow the
other one is a secondary rainbow now
difference between I mean both of them
look like the normal rainbow which you
have ended up swinging on the sky
the difference between a primary rainbow
and secondary rainbow is because of the
difference in their coordination because
of the difference in the way they are
formed they are named as primary rainbow
and secondary rainbow so now we will
look at how is the rainbow formed so let
us first look at the primary rainbow
formation how this apply with a rainbow
font it is a stepwise process so what
happens now generally as I mentioned a
rainbow is generally formed after a rain
right so when there is a rainfall which
had taken place that means there will be
some raindrops already present in the
atmosphere right some water droplets
will be present in the atmosphere now
what happens but and I also mentioned
that it is warm only when some part of
the skies is sunny that means
sunlight is present so what happens when
this sunlight enters the raindrop
diffraction it feels as you can see in
this figure this is the sunlight which
is entering the raindrop
so refraction will take this now
sunlight is again white light so as it
enters the raindrop it gets disposed to
do the different colors so you can see
that the red end and the violet and get
deviated by different angles because
they have got different wavelengths and
they are entering a medium of different
attractive index like so refraction of
sunlight takes place in the next step
what happens total internal reflection
of light takes place inside the rock now
when they reach this bound to be what
happens
total internal internal reflection takes
place that means they get internally
reflected and then they again come out
from the other end so what happens here
again refraction of the reflected light
takes place as it comes out of the drop
because again there is a change in
medium right so what happened
by entering inside the raindrop there
was a refraction inside there was
internal reflection vennett's when it
strikes the boundary of the raindrop it
comes back it bounces back that is total
that is internal reflection and then
again when it comes out from the
raindrop to the atmosphere again
refraction takes place so as a result of
this what happens let us suppose if you
are the observer so it's later suppose
if you are standing somewhere here so
what do you see you see the light rays
which are coming out of the raindrop
right for example when I was talking
about refraction through a glass slab I
told you that you I mean the observer
always sees the emergent rate that is
observer always sees the ray which is
coming out of the medium so what is that
emergently in these case these raising
rays which are coming out of the medium
are the ones which VC so the observer
sees the Constituent colors right
because that in what was the input to
the raindrop input was white sunlight
that is white light but what was the
output output is the spectrum of
different colors because now dispersion
had already also taken place like so
that so what does the observer see
firstly the observer will see the
dispersed like that is it sees all the
seven different colors right from all of
the raindrops so as a result we are able
to see a rainbow so now you can
understand that why don't we see a
rainbow after the rain because only
after the rain you have so many water
droplets in the atmosphere why do we see
it then one part of the sky is sunny
because only then you will have sunlight
and sunlight and rain drops are the two
necessary things for a rainbow formation
and why do you see why it why are you
able to see a rainbow only when you
stand keeping your back towards the Sun
that is because if you see sunlight
comes from here and it falls on that a
draw and then what is whatever is coming
from the raindrop if that reaches your
eye only then you will be able to see
the rainbow so if you stand
such a way that your back is towards
these raindrops that means your front
you are facing the Sun in that is you
will not be able to see the rainbow
because these rays coming out of the
raindrops will not be able to reach your
eye right let us now look at the
formation of secondary rainbow day to
see what is different in case of
secondary rainbow in case of secondary
rainbow also a refraction of sunlight
takes place as it enters the raindrop
internal reflection also takes place so
here if you see this is how the sunlight
enters this is how eternal reflection
takes place but in this case internal
reflection takes place inside the drop
for the second time so it is it is
something like there are some somewhere
crack I mean somewhere internal
reflection will take place only once
somewhere internal reflection will take
place twice but however the rain move
which you see will remain the same there
is no difference in the view of a
primary rainbow and the secondary
rainbow it is just that since during the
formation of a secondary rainbow the
internal reflection inside the raindrop
takes place twice so it is known as a
secondary rainbow and again when it
comes out of the drop refraction takes
place as again there is a change in
medium so rest everything remains the
same it is just back in case of a
secondary rainbow the sunlight enters
the drop in such a way that it strikes
the boundary of the drop twice and
bounces back so internal reflection
inside the drop takes place twice so
that is the only difference so I hope
you are able to understand that when is
a rainbow form by gives a rainbow formed
at that time and how is a rainbow form
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