L5 : Electromagnetic waves - Electromagnetic Waves, Physics, Class 12 Video Lecture

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before going ahead with part 5 now I
told before also that electromagnetic
waves are nothing but coupl time varying
electric and magnetic fields that
propagate in space now let us suppose
that if you have an electric field which
is changing with so what do we mean by
electromagnetic waves as I mentioned
just before that the electric field
there is one electric field which is
varying with time or which is changing
with time so this electric field will
give rise to a magnetic field right now
the magnetic field again which is just
now produced is again bearing with time
so this magnetic field will again give
rise to an electric field that electric
field is again varying with time so it
will give rise to a magnetic field so
what are we saying electric field
producing magnetic field that magnetic
field again producing electric field and
that electric field again producing
magnetic field and this process
continues
so these electric and magnetic fields
are known as tiny burning and coupled
coupled means they are related to each
other one is giving rise to the another
to such electric and magnetic fields
when they propagate together in space
they give rise to electromagnetic waves
now here if you see the blue line
represents electric field let us suppose
if we have any particle whose electric
field varies like this in the form of a
sine wave
now this electric field will give rise
to a magnetic field which will also be a
sine wave but in a perpendicular
direction to electric field that means
let us suppose this is my electric field
and the magnetic field is somewhat like
this in this plane right so that is why
I have represented it like this because
it is
other plane I mean you can imagine it
somewhere Tigers when you imagine your
electric field it is up right when you
imagine your magnetic field it is
horizontal lying down that means it is
in a plane perpendicular to the electric
field so these two together propagate in
space so the entire wave propagates in
this direction now let us suppose it
electric we now this electric field
magnetic field and the wave propagation
they are all perpendicular to each other
now if the electric field is along x
axis and the magnetic field is along Y
axis then the wave will propagate in the
z axis right because one way electric
field let us suppose the electric field
is along x axis that means the electric
field is moving along this direction
magnetic field is along Y axis that is
this direction so your wave will
propagate in the z axis so in this case
the wave propagates along this direction
right so this is the direction of
propagation of wave so that electric
field and the magnetic field will be
perpendicular to each other as well as
perpendicular to the direction of wave
propagation you understood so electric
and magnetic fields which are time
varying and coupled to each other give
rise to an electromagnetic wave so now
we will talk about sources of
electromagnetic waves what gives rise to
electromagnetic waves I mean what can be
a source from where I can get such kind
of electric field such that very
electric field which can give rise to a
magnetic field and the magnetic field
again can give rise to an electric field
and so on so forth so what can be such a
source accelerated charges radiate
electromagnetic waves so accelerated
charges are basically the sources of
electromagnetic waves why so because
when a charge is accelerating the
electric field associated with the
charge will also change accordingly
right because if the let us suppose if
the charge is at rest in that case the
charge is not moving at all so that
means the electric field associated
the charge will remain static the
electric field will not change with time
so if the electric field is not varying
with time it will not give rise to
magnetic field so there is no way of
generation of electromagnetic wave
similarly if you have a charge which is
moving but it is moving with uniform
velocity that means the acceleration is
zero so in that case also what will
happen to the electric field the
electric field the change in electric
field with time is also constant the
electric field is not varying with time
now if it doesn't vary again there is no
way you can generate electromagnetic
waves so axillary discharges can a lone
chair radiate electromagnetic waves so
let us take the example of an
oscillating charged particle then you
think of an oscillating charged particle
you always get an idea of the pendulum
this is a charged particle which is
oscillating now if the particle is
oscillating that means it is a charged
particle so it will have an electric
field associated with it so what will be
the electric field the electric field
will be an oscillating electric field
now this electric field will give rise
to an oscillating magnetic field now
this mosque oscillating magnetic field
in turn will give rise to an oscillating
electric field right because since the
magnetic field is oscillating that means
the magnetic field is changing with time
now it is changing with time so it gives
rise to an electric field so oscillating
electric field gives rise to oscillating
magnetic field oscillating magnetic
field again gives rise to oscillating
electric field so and this process
continues
so regional so as a result this
regeneration is like propagation of wave
so like electric field from magnetic
field again man electric field from
magnetic field magnetic field from
electrically so this will again give
rise to oscillating magnetic field so
this regeneration of electric and
magnetic fields so we can say that
regeneration of electric field and
magnetic field is like propagation of a
wave because we know that propagation of
wave is what it is nothing but a field
which tells us that the wave is moving
in this direction but actually is it the
wave which is moving it does not the
wave it is basically the disturbance
right we have studied all these things
in our basics of wave in the lesson on
waves in class 11th right so here also
this oscillating electric field
producing magnetic field again magnetic
field producing electric fields and so
on so forth this entire phenomena gives
us a field or it gives a feeling as if
the wave is propagating in a particular
direction and this wave is known as
electromagnetic waves so therefore we
can say that the frequency of the
electromagnetic wave is nothing but the
frequency of this oscillating particle
so we can say that frequency of
electromagnetic wave is equal to the
frequency of oscillating particle right
okay so this is what we studied about
the sources of electromagnetic waves now
let us talk about the nature of
electromagnetic waves like when I talk
about nature I mean what kind of waves
are they they are transverse in nature
or they are longitudinal I have spoken
about we have already discussed in very
much detail the different types of waves
that is the transverse waves
longitudinal waves so here the
electromagnetic waves electromagnetic
waves are transverse waves now let me
quickly remind you water transverse
waves and longitudinal waves transverse
waves are those in which the particles
move I mean the each particle of the
medium they move in a direction
perpendicular to the direction of
propagation of wave for example here if
you look at this figure this is the way
this is a transverse field so what is
the direction of propagation the
is the direction of propagation but if
you concentrate on each of the particle
the wave is moving somewhat like this so
if you can't concentrate on each of the
points of the medium each particle of
the medium is moving along this
direction so that means the particles of
the medium are moving in a direction
perpendicular to the direction of wave
propagation so they are transverse waves
when I talk about longitudinal wave it
is somewhat like this here if you see
that what is the direction of wave
propagation the wave propagation is
along this direction if you look at the
overall picture you see that the entire
wave is getting displaced towards the
right now what is the direction of
motion of each of the particle of the
medium now concentrate on any one
particle so example I have chosen this
particle so what do you see if the
particle is oscillating to one flow in
this direction in the horizontal
direction itself so that means in this
case the particles of the medium are
oscillating along the direction of wave
propagation so these are longitudinal
waves and this is transverse wave so now
when I talk about electromagnetic waves
electromagnetic waves fall under this
category that is electromagnetic waves
are transverse in nature
here if you see the wave property if the
wave propagation takes place along x
axis the electric field and the magnetic
fields they are at perpendicular
direction to the direction of wave
propagation so let us suppose if the
wave propagates along x axis then the
electric field will be along Y axis and
the magnetic field will be along z axis
that means both the electric field will
be perpendicular to the direction of
propagation and the magnetic field will
also be perpendicular to the direction
of propagation so that is why and that
is how electromagnetic waves are
transverse waves now let us stop about
so how can we represent the electric
field and the magnetic field in an
electromagnetic wave it is very similar
to how we represented in piece of
general wave so we can say that electric
field is given as e naught sine KX minus
Omega T let us suppose that the wave
propagates along x axis so this would be
K X minus Omega T and the electric field
is along Y axis and how do we represent
the magnetic field it will be B Z is
equal to B naught sine KX minus Omega T
because this X represents the direction
of propagation of the wave and what is K
ki is nothing but the wave number or
often known as the propagation constant
so wave number is given by 2 pi by
lambda where lambda is the wavelength of
the wave now all these things I have
already discussed in the lesson on waves
that is why I am NOT repeating them
again all right so this is how we
represent electric field and magnetic
field it gives off an electromagnetic
wave so these this will be helpful in
solving problems so please understand
this and remember them right so thank
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