Spherical Mirrors Video Lecture - Physics Class 12 - NEET

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reflection and diffraction Factory is
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please make sure that you have watched
all the videos to the part two before
going ahead with part three so now that
we have talked about what is a
reflection of light let us talk about
spherical mirrors what are spherical
mirrors as the name suggests what do you
think what can which we it has to be a
mirror right spherical that means maybe
the middle is spherical in shape correct
that that is the only possibility which
we can assume from its name right so
spherical mirror is a part of a
reflective spherical surface that means
if you look at this picture here you
have a big sphere which is made up of
which is nothing but a mirror I mean you
can imagine it is something like it is a
sphere you can imagine any sphere like
your football but that football is a
kind of a middle I mean you can see your
image on that here you can see that the
people who are standing and they can't
see that image is on this way so
spherical mirror is a part of a
reflective spherical surface that means
it is a spherical surface which is
reflective that means which is capable
of reflection right okay now there are
various types of spherical mirrors the
major two types of spherical mirrors are
concave mirror and convex mirror so
these are the two types of spherical
mirrors concave and convex so which is
what is a concave mirror concave mirror
is the one which is silvered on the
inside of sphere so generally when we
talk about this spherical mirrors these
very I mean the mirrors which you see I
mean you you they are concave mirrors
and convex mirrors are something which
we use in our day to day life but they
are not exactly in the shape of a sphere
it is basically a part of the sphere for
example you see this mirror right it is
in the form of a sphere let us suppose
this is a sphere so you just take
part of the spherical mirror so what
will happen it will look somewhat like
this it is a part of that entire
spherical surface right so spherical
mirror is basically a part of that
entire spherical surface which is
reflective in nature right so when I
talk about a concave mirror it is
silvered on the inside of fear that
means if you see this is my concave
mirror wherever silvered means which
which portion is called silvered Silbert
is the portion where the middle actually
is I mean in front of which you stand
for example at your home you have a
mirror right when you place the mirror
in front of you the front part of the
mirror where you can see your image that
is the silvered side of the mirror so
when I talk about the concave mirror the
inside sand surface of the sphere is
silver as you can see here this portion
is silver inside part is silver so the
other part that is the back of the
middle is denoted by these lines this is
how we represent a concave mirror so
that means the the person whoever is the
or the object has to be present on this
side so that you this is your silver
torsion and you can see your image right
similarly when we talk about a convex
mirror it is silver on the outside of
the sphere right so in this case if you
see if this this is the PUF this is a
part of a sphere right so the outside
portion of the sphere for example this
is a sphere right so in case of convex
mirror the outside is silver that means
the inside is not silver so this portion
is so if you look at the diagram which I
have here dissimilar to that right so
the outside portion is silver so the
object or the person is standing on this
set so you understand what is a fine
cave mirror and what is a convex mirror
so let me repeat once again spherical
mirror means it is a part of a spherical
surface which is reflective in nature
that means which is capable of
reflection there are two types of
spherical mirrors concave and convex
when I talk about the concave mirror
it is silvered on the inside of the
sphere when I talk about convex mirror
it is silvered on the outside of the
sphere right so it is very important to
remember which is a concave mirror and
which is a convex mirror please do not
get confused with this so you can use a
memory tic to remember what is which one
is concave and which one is convex if
you remember this word if you look at
this word concave you have cave in it
cave means all of you know right what is
a cave cave means you call it as what
you call as Buffon right so cave means
when you enter inside something so that
is what the inside is feared so wherever
you have this word cave you have inside
that means the inside is say bird so
that is fun care and the other one that
has fun vex
is outside the silver so now let us look
at some of the terminologies related to
a spherical mirrors because now we are
going to study about I mean our
objective right now is to study about
reflection of light by spherical mirrors
I mean how reflection takes place by
spherical mirrors and how image
formation takes place in spherical
mirrors for that we should be aware of
some terminologies related to the
spherical mirrors so the first term is
pole
what does pole of a spherical mirror it
is the centre of the reflecting surface
so let us suppose if I have this
spherical mirror which mirror is this
whether it is upon Kaiba convex the
inside is silvered right so inside means
cave that means it is a concave mirror
so center of the reflecting surface this
is my reflecting surface so the center
of this reflecting surface is called
pole that means this point is pole it is
generally denoted by a capital P the
next is centre of curvature which is
denoted by a capital C it is the centre
of the sphere of which the mirror is a
part as I mentioned before also
spherical mirror is part of a spherical
surface so if you look at it this spell
also a part of some sphere like this
correct so the center of this sphere is
known as the center of curvature that
means for this mirror this is the center
of curvature that is denoted by a
capital C next is radius of curvature so
ready family it should be on similar
lines as centre of curvature so radius
of curvature is the radius of the sphere
of which the mirror is a part that means
radius of this sphere so this radius
would be known as the radius of
curvature next is principal axis it is
the line joining the pole with the
center of curvature that means this line
is known as principal axis so you
understand the difference
this distance is radius of curvature but
this line is known as the principal axis
so the principal axis and extend this
way so this line is called as principal
axis but the distance is this distance
that is the distance between the center
of curvature and pole is known as the
radius of curvature right okay the next
one is principal focus it is a very
important term and it is a very
important concept as well so what does a
what is principal focus principal focus
is a point on principal axis where the
rays of light incident on the mirror
parallel to the principal axis converge
or appear to converge so there is
something important to understand here
so first part you understood that
principal focus is a point of principle
axis okay
what happens then let us suppose for now
I am considering a concave mirror let us
suppose this is a concave mirror so in
your books you have representation like
this so this blue region is same as this
right so this is your concave mirror
these are the incident Ray's right
because these rays are in sea
on the mirror so they are incident rays
and what are my reflected rays after
striking the mirror they are getting
they are bouncing back so these are my
reflected rays okay so it says that
principle focus is a point on principle
axis where the rays of light incident on
the mirror parallel to the principal
axis so where is my principal axis this
is my principal axis so if you see the
Rays incident rays are parallel to the
principal axis when they fall on the
mirror after reflection there is a point
where they converge so they are all
meeting at one point so this point is
known as the principal focus and it is
generally denoted by a capital F this is
my principal focus so this point was
pole
this point was centre of curvature and
this is now principal focus right so
what is principal focus it is the point
where the incident rays which are
parallel to the principal axis after
reflection converge or appear to
converge so in this case in case of
concave mirror the rays of light
actually converge then why did we use
this term appear to converge that is the
scenario for convex mirror because in
case of convex mirror what happens when
incident ray is parallel to the
principal axis strikes the mirror they
get reflected in this way I mean they
get separated away from each other here
what did you see after reflection all
the reflected rays came towards each
other they all converged to a point but
in this case these are my reflected rays
right so these reflected rays if you see
they are going away from each other
so it doesn't look like that these rays
will ever meet with each other but if
you extrapolate it towards the back you
see that they they are not actually
converging here right because they are
moving away from each other but they
appear to converge at this point so for
convex mirror we define this point as
the principal focus so that is why when
we write a general definition for
principal focus we
that it is the point or the principle
axis where the rays of light incident on
the mirror parallel to the principal
axis after reflection either can
converge as in case of concave mirrors
or take appear to converge as in case of
convex mirror so that means in case of
concave mirror the reflected rays are
actually converging at the principal
focus but in convex mirror the reflected
rays are actually diverging but they
appear to converge at or they are
virtually converging at the principal
force right please understand this slide
very clearly because understanding
principle focus is a very important
concept so now let us see one another
important thing so just now I told
something very important in case of
concave mirror what happens the incident
rays have a tendency that after
reflection they will converge at a point
and that is why concave mirror is also
known as the converging mirror because
it has a tendency to converge the rays
after reflection whereas in case of
convex mirrors what happens the incident
ray after striking the mirror they tend
to diverge away from each other and that
is why this is known as a diverging
mirror so often it is asked in your
question that which is a converging
mirror concave or convex so you should
remember all these things thank you
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