L13 : Concave & Convex Lens - Light, Science, Class 7 Video Lecture

hello friends this video on light part
13 is brought to you by example comm no
more fear from exam so now we will talk
about another important thing now have
you ever observed your grandparents or
any other elderly person wearing
spectacles in fact these use a lot of
people who are young they also wear
spectacles so what are these spectacles
the glasses which are there in the
spectacles what are those glasses are
their mirrors they have got mirrors
because in mirrors you have one shiny
surface the other surface is not shiny
like how you have your the mirror of
your dressing table so on one side you
have glass when the other side is not
glass but when you look at this glass of
the spectacles you have glass on both
the sides so it doesn't look like a
mirror then what is it
so this is not only this think of
another example let's say you have a
magnifying glass now which often the
astrologers used use by looking at your
hand will cuneta pan or which is often
used to magnify any object there also
you will see that both sides are glass
so again that is also not a mirror then
what it is so these are nothing but
these are got lens so it is a lens and
not a mirror now you learn about more
about lens because lens is another
important thing which may I probably use
of the properties of light and we will
see that the way we have different types
of middle similarly we have different
types of lenses as well and these lenses
they help us to achieve a lot of purpose
so here we will be spending some time in
order to understand lenses so basically
your brand-new spectacles they are made
of lens and not mirrors similarly the
magnifying glass to read they are very
small prints or to see very small
objects is also made above lens so image
formation happens not only through
mirrors but also with lenses so that's
what we will study now
so what is the lens it is a transparent
optical medium bounded by two surfaces
at least one being spherical so like in
case of mirror you had just one surface
which is smooth and shiny and where a
reflection happened here instead of one
you have two surfaces so something when
you look at the lens of a spectacle
that's the best place where you can
actually get an idea about how a lens
feels to me so on both the sides you
will see there are two they're two
different surfaces it is not one surface
now out of these two surfaces one has to
be spherical you think this is how the
lens actually is so it is like a
magnified version of a lens so can you
see here that it is like bunching and
the inmate at the middle and it is like
kind of tapered towards the end so
that's how the shape is so this is a
type of lens now it is not always
necessary that both the surfaces have to
be spherical now I will give you some
exactly for example let's say that one
selfies could be concave the other
surface could be plane one sense this
could be concave the other senses could
also be a concave one selfies could be a
convex are that could also be a convex
and so on so basically there are many
different types of lenses so let us have
a quick look you see by convex
by means two so both the surfaces have
contents similarly by concave both the
surfaces are concave right you remember
concave means the inner surface is
smooth so here you see the inner surface
is smooth again here also the inner
surface is smooth and both of them are
joined together and that's why inside
what you have is glass similarly here
the outer surface here and the outer
surface here so it is by convex
similarly you can also have plan of
convex where one side is plane and the
other side is convex you can also have
plan of concave where one side is plane
and the other side is concave so these
could be various combinations
of lenses and again the image formation
with each of these types of lenses are
going to be different now when we talk
about types of lenses again just like
mirrors there are two types of lenses
convex lens and concave lens now one
very important thing that I would like
to tell here is you will observe in a
little while so one interesting thing
that will be observed is that the
behavior of the convex lens to a large
extent is similar to that of concave
mirror so the names are just the reverse
concave mirror and convex lens so they
have a lot of similarities because both
of them are the converging mirrors and
converging lens respectively so let's
talk about cont convex lens now which
one is a convex lens anything which is
thicker at middle than the edges so you
see it is thick at the middle and thin
at the edges so this is a convex lens
and it is just the reverse in case of a
concave lens then otherwise also you can
see that here you have the convex
surface that is the outer surface open
on both ends and here it is just the
reverse again so these are the two types
of lenses that we will be talking about
now the way we have discussed about the
image formation with different types of
mirrors similarly we are going to do the
same for lenses so let's first talk
about convex lens so convex lens is a
converging lens you remember this is
true for concave mirror so concave
mirror is also converging and convex
lens is converging so an rays of light
pass through it after refraction now
with lens and second case of mirrors
reflection happens the libraries get
reflected now in case of lenses so when
you talk about mirrors we talk about
bouncing back of light that is the
reflection of light but when we talk
about lenses so we do not talk about
reflection we talk about another
phenomenon of
I'd call the refraction of light now
refraction is something which you will
be learning in your higher classes but
just for your information this is not
bouncing back of light rather than
bending of light as it enters from one
medium to another so here as you can see
the medium what is the medium it is air
and here what is the medium it is water
and again here what is the medium it is
air so as the light rays move from air
to water it bends again as it moves from
water to air it bends so that is known
as the refraction so whenever we talk
about lenses we talk about refraction so
here with the convex lens we see that
there is after refraction they all meet
at a particular point so that is they
converge at a point they meet at a point
that is why it is a converging lens and
it is just the opposite in case of
concave lens because here after
refraction they all move away from each
other so they get scattered
so they diverge so that is why quantity
of lens is a diverging lens however
these diverging refracted rays appear to
be meeting at a point which is this
point so this point is nothing but these
points are the principal focus of the
respective mirror of the respective
lenses thank you
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