Power of a Lens Video Lecture - Physics Class 12 - NEET

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reflection and refraction part 18 is
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17 before going ahead with part 18 so
let us now talk about magnification of a
lens I think now it is easier for you to
understand what is magnification what is
lens formula
how is image formation taking place in
lenses because we have studied all these
things in detail for measures right so
what does magnification of a lens it is
also the same thing it will tell us how
big or how small the image is compared
to the object so it is the ratio of the
size of an image to the size of the
object it is generally denoted by a
small M now whenever we talk about
direct images force magnification is
positive that is what very obvious
because magnification is energy - by H
so if both the image if the image formed
is erect that means it is above the
principal axis so it will be positive
for inverted images M is negative let us
now talk about power of a lens power of
a lens is the measure of convergence or
divergence of a lens whenever we talk
about power of a lens it means that what
is the capability it is a measure of the
capability of the lens to converge a
beam of light or to diverge a beam of
light that means let us suppose if I am
talking about a converging lens how much
it can converge to what extent can it
converge a ray of light that is measured
by the power of a lens you would have
heard from people that those who have
got some defects in their vision they
usually use lenses right instead of
spectacles many of them use lenses these
days that lens has some power you did
somebody wearing a spectacles must be
knowing that use of ends of -1 + 1 let
that
have different powers what is the power
it tells us the ability of the lens to
what extent it can converge or diverge
and incident beam of light so this power
of the lens depends on the focal length
of the lens so we will prove that how it
depends on the focal length of a lens
let us suppose we have this lens where
you can see the second principal focus
lines here so that means F is the focal
length for this lens now let us suppose
I take a similar lens with a greater
focal length let us suppose the second
principal focus line somewhere here so
in this case Oh F is greater than the
wave in this case so what will happen in
this case when a ray of light is
incident parallel to the principal axis
it has to pass through this focus so
what happened in this case in this case
they are bending reduced the value of
this Delta dash became smaller than the
value of this Delta similarly if this F
would have been even more far away in
that is this wave would have been
somewhat like this right so that means
your Delta would have further decreased
to some Delta dash - that means as the
focal length of the lens increases the
angle by which it converges that
decreases that means as focal length
increases your Delta is decreasing as
focal length increases Delta decreases
so as the focal length increases the
degree of convergence decreases so we
can see that in this degree of
convergence denotes power so we can say
that power is inversely proportional to
the focal length of a lens so therefore
we define power mathematically as
reciprocal of focal length of a lens so
we define it as the tangent of the angle
by which it converges or diverges
a beam of light falling at unique
distance from the optical center so this
is how it is officially define this is
the tangent of this angle that this time
Delta by which it converges
a beam or light falling at unit distance
from the optical center that means when
these distances one unit so this is how
it is officially you find power when I
talk of a convex lens power of a convex
lens is positive why because power is
equal to 1 by focal length and focal
length 4 upon fixed lens is positive so
mean any power of a concave lens is
negative that is because focal length 4
upon cave length is negative SI unit of
power is dioctyl which is denoted by a
capital d one diopter is equal to one
meter inverse because it is SI unit of
power that is one guy optic is equal to
one divided by what is the SI unit of
focal length it is meted so 1 diopter
becomes equal to one meter inverse thank
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