Reflection of Plane Waves using Huygens Principle Video Lecture | Physics for ACT

hello friends this video on wave of its
part 8 is brought to you by its anthem
no more fear from exam please make sure
that you have watched all the videos to
the back seven before going ahead with
part 8 so now we will try to understand
the phenomenon of light like the
reflection of light as well as the
refraction of light using Huygens
principle so that we get an even better
understanding of Huygens principle
because what happened down here was when
you doesn't give the particle theory of
life he explained these phenomena
because of which his theory was accepted
you can explain the phenomenon of
reflection and infraction using the
corpus critical so that is how so
therefore if you want to use the truth
of the icons principle you will have to
prove the same phenomena of reflection
and reflects refraction using Huygens
principle so let us see how good the
Huygens principle could explain the
reflection of plane waves so in this
case a good observation is necessary so
let us now study a reflection of plane
waves using items principle so here also
the same concept the concept will remain
the same just that I'll show you how the
phenomenon thing is let us suppose in
this case how will it is because this is
the boundary these are my incident waves
so incident waves would be somewhere
like this so how will the wavefront of
this incident wave look like if the
waves are coming like this the points of
same phase when dotted together the
incident the way it will be somewhat
like this so this will become the
wavefront for the incident beam so this
is my incident wave front right so now
when the incident rays fall like this
after reflection they will go back
somewhat like this so how will they
reflect reflected wavefront look like
they reflected wave Frank will look
somewhat like this right so this is my
incident to be in friend and this is my
reflected wave friend because when we
are studying reflection using hyejin's
principle we will understand it in terms
of wave friends because items consumer
talk only in terms of wave fronts it
doesn't talk about light rays or light
waves okay so this is my incident wave
Frank and we will see the behavior of
this incident wave front when it
interacts with this boundary now what
happens this is my incident wave front
how will it interact with the boundary
see think of the situation we should try
to visualize the situation suppose a
bunch of rays are coming in this way
then all of them will not strike the
surface together right there is which
are lying here and they will touch the
surface before the rays which are here
right so that means the rays which are
here it will touch the settings after
some time D from the time when these
rays touch the surface that means this
wavefront will interact with the surface
gradually let us suppose that the time
taken by this portion of the wavefront
to interact is tau so tau is the time
taken for this wavefront to interact now
what happens when the interaction
actually takes place now as soon as the
incident wave or the this incident wave
friend interacts with the boundary what
happens the wavefront gives rise to
secondary wavelets as mentioned before
that means from every point of this
wavefront secondary wavelets will arise
when you interact with the boundary
right so this wavelet because now you
might ask why business secondary
wavelets arise only when it interacts
with the boundary because only when some
disturbance is there only then the
wavelets will arise right for example if
we do not throw the stone into the water
at all with the secondary wavelets form
it will not form because there is no
source of disturbance right so there
has to be some source of disturbance so
here that source is the interaction with
the boundary so whenever the incident
wave length wave front will touch the
boundary then every point of the
incident wave front will give rise to
secondary wavelets now what will be the
secondary wavelets for example if this
point of the incident wave front touch
the boundary then secondary wavelets
will arise in this fashion right so what
will be the wave front for this it would
be a sphere like this similarly let us
suppose that after time T one for the
same situation that is suppose this
incident wave is now here at this point
earlier it was at this point now it is
at this point so what will happen again
it will give rise to waves like this so
this will be the wave front now earlier
also there was one more different so
gradually and what will happen what will
be the radius of the wavefront
in this case what will be the radius of
the wave length here the radius of the
wavefront will be nothing but the
velocity that is the speed of the wave
which is B into the time taken what is
what will be the time taken time taken
is nothing but this time tau so this
will be V into tau now gradually as it
moves this side that means as each of
these points interact what will happen
time tau will keep decreasing because
tau is the time taken to reach from here
till here now when you are already
reaching here when the point here is in
interacting with the boundary it is less
than tau that means gradually the radius
of the sphere will keep decreasing right
so then locus or the common tangent to
all these spheres gives rise to the
reflected wave front because reflected
wavefront is nothing but the incident
wavefront position of the incident wave
friend after time tau right incident
wavelength we know we want to find out
the position of the wavefront after
some kind top so that way friend will be
nothing but the reflected wavelength
correct understand please understand it
properly so ray of life's falling on the
surface is nothing but they concede and
wavefront so we know the incident wave
friend we want to find out the reflected
wavelength so what is reflected
wavefront it is nothing but the position
of the incident wave front after some
time tau so what do we say we say that
every time each point on this incident
wavefront in clash with the boundary it
gives rise to secondary wavelets now we
will draw the wavefront for the
secondary wavelets and a common tangent
which joins all these secondary wave
fronts is nothing but the position of
the reflected wave front like so let us
look at this animation which will help
you to understand it even better so here
this green wave which you see this green
wavy present nothing but the incident
wave front
this represents the incident wave front
please understand incident wave Frank
and incident light waves are two
different things
if ins this is the incident wave front
that means that the light waves are
falling on the surface like this right
that is why this is the wave front
that's what I explained you here right
so what happens here now as you see as
soon as this incident wave front touches
the surface everywhere it gives rise to
one secondary waves and the line or the
common line which joins the joints all
the secondary wavelets is nothing but
the reflected wave vector right so this
animation is it is just a rough
animation just to help you understand
how the reflection takes place or how do
you determine that what will be the
reflected wave front using the Huygens
principle so I hope it is clear to you
okay so now let us look at it
diagrammatically
here we are only talking about
reflection so let us suppose the my
incident waves are falling on the
surface like this so therefore my what
will be my incident wave front this will
be my incident wave front so that is why
this is my incident wave front
this is incident main front right now
every at every point it gives rise to
secondary wavelets right and if you draw
a common tangent to all the secondary
wave friends what do you get is nothing
but this reflected wave now how do you
calculate what would be the radius of
each of these spheres that we do this
way let us suppose that the speed of
wave is V right so then if we call this
point as PE so here in this diagram we
have also depicted the same thing here B
is nothing but the incident wave front
and see E is the reflected wave right so
how much time will it take for B to
reach C that means how much time will it
take for this particular portion of the
incident wave front to cover this entire
distance let us suppose that time is tau
so what would be BC that means how much
time you take for point B to reach point
C that will be nothing but velocity into
time taken that is V tau that means at
Point a when this point E interacts with
the boundary it will give rise to
secondary waves so what could be the
radius of this wavefront it will be
nothing but V into tau right so that
means this wave which a will form this
will have a radius which is given by V
into town now after some time this
incident wave trying to will interact at
this point at that time tau will keep
decreasing therefore at every point as
you go towards
the radius of the sphere or the radius
of the wavefront will keep decreasing
and then we draw a common tangent which
is nothing but the reflected wavelength
then so I hope it is clear to you that
how do we explain a reflection of
playing waves using Huygens principle
similarly we can explain the refraction
of waves using Huygens principle thank
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