L9 : Radioactivity - Nuclie, Physics, Class 12 Video Lecture

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till part 8 before going ahead read part
9 or 10th topic that is called
radioactivity now since we are talking
about nucleus in this lesson so it is
very important that we talk about
radioactivity why because as I mentioned
when we plotted the curve for binding
energy per nucleon with mass numbers
what did we see we saw that when the
mass numbers becomes very high for very
high mass numbers the nucleus becomes
unstable and those unstable nuclei show
a phenomenon which is known as
radioactivity and this phenomenon has
many practical applications which we
will discuss in the later half of the
chapter so to start with let us see what
is radioactivity since it is a
phenomenon which is shown by nucleus
therefore we are studying it in this
lesson so what is radioactivity it is a
nuclear phenomenon in which an unstable
nucleus undergoes decaying to form
stable nuclei that means you have you
remember the plot of binding energy per
nucleon where I told you that for
heavier nuclei since the binding energy
is less the nucleus is less stable so
what it does it splits into lighter
nuclei which are relatively more stable
let and that is how it tries to gain
little stability so this so such kind of
phenomenon is basically referred to as
radioactivity
so this radioactivity is also a nuclear
phenomenon in which an unstable nucleus
undergoes DK to form stable nuclei now
exactly which nuclei I mean isn't that
all nuclei will show radioactivity no it
is not like that only certain nuclei
which are unstable will show these kind
of phenomenon this phenomenon was
discovered by a scientist named Henri
Becquerel
somewhere around 1896 now--how Becquerel
discovered this phenomenon was also
quite interesting so it was a kind of
accidentally that Becquerel was able to
discover this phenomenon of
radioactivity he was actually working on
a different topic he was basically
trying to perform experiments on
fluorescent objects like so if he
believed he had this hypothesis that
there are certain fluorescent objects
and before that let me tell you what is
fluorescence what do I mean when I say
fluorescence it means that when light
falls on an object that object will also
emit some visible light let us suppose
you have an object here so when visible
light or when sunlight falls on it this
object will also emit some light so this
property of this object is called
fluorescence and the object is said to
be a fluorescent object now they can I
mean have this hypothesis he thought
that such objects they emit visible
light but along with visible light they
emitted some other kind of radiation as
well but he didn't know what kind of
radiation it was in fact he was not even
sure whether they emitted some other
kind of radiation or not it was just a
hypothesis which he had he assumed that
if they emit some other kind of
radiation as well now in order to see
this experimental you he had an
experimental setup what did he do he
took a photographic plate he placed it
inside a box like this and then he took
some object for example he took a metal
right and then he allowed this Rays of
the Sun you fall on this letter now
since this box inside which the
photographic plate is inked the box is
actually opaque so the box will not
allow the Rays of the light to pass
through it therefore when the
photographic plate is developed there
should it should be
there should be no traces on the
photographic plate because the sunrays
will not reach the photographic plate at
all
now when you placed this object he took
some fluorescent object and placed it
over this box so now what will happen
the when life will fall on this object
if the object is fluorescent it will
give out the light it will give out
again some light then what did it he
again took out the plate photographic
plate which was placed inside the box
and developed it and he saw that there
the trace was normal I mean the
photographic plate was long but there
was no image or no mark seen on the
photographic plate so now he tried this
experiment with different objects right
now instead of first it took some metal
then he took some other object and again
perform the same experiment so when he
developed the film again you saw that
there were no spots found on them Phil
so what was it proving it was proving
that there is the the sunlight is
falling the sunlight is the only ray
which is getting emitted here and since
the plate is covered with an opaque
object therefore the sunlight is not
able to reach the plate right so if he
was thinking that maybe his hypothesis
is wrong so he was trying this
experiment with many different compounds
he tried with some metals he tried with
some other kind of elements some other
compounds as well but everywhere you
were seeing that the result was turning
out to be almost the same then he took
some a compound of uranium that was
called uranium potassium sulfate so this
time he observed something different so
he used uranium potassium sulphate so
this was the substance which he used and
then what did he observe now this time
when he developed the photographic plate
he saw a black spot on the photographic
plate there was some spots present on
the photographic plate what does this
mean this mean
there has been some radiation emitted
from somewhere which could reach this
photographic plate so now his question
was from there was this radiation
emitted left because he was not able to
understand whether this radiation was
emitted because of the presence of the
sunlight or the reaction of the sunlight
with this compound or since this
compound was also made up of different
elements like it had uranium it had
potassium with had sulphur so he
actually didn't know from where did this
radiation generated so for that in order
to get an answer to this question of his
what did he do he kept a blade over the
box because earlier just above the box
he kept the compound that is uranium
potassium sulfate this time he kept a
blade and above this he placed uranium
potassium sulfate and then he repeated
the same experiment so this time when he
developed the photographic plate to his
surprise he saw that the pattern of the
blade was seen on the photographic plate
that means the radiation was emitted by
this object so the radiation was emitted
by the uranium potassium sulphate so he
became sure of one point that who or who
generated the radiation so now he knew
that now his next question was is it
because of the presence of sunlight that
this radiation got generated or did this
radiation can be generated even in
absence of sunlight so now what happened
while he was performing this experiment
what while he was performing this
experiment I suddenly accidentally it so
happened that the day was not that sunny
so he had to stop his experiment because
till then he was assuming that sunlight
was something which is mandatory for him
to carry out his experiment so what did
he do when a day when there was no
sunlight he perform the same experiment
and to his surprise this time also the
result which he obtained was the same
that means this time also when he
developed the photographic plate he
could see the
image or the same impression of the
blade coming on the photographic plate
so now he concluded that the radiation
was emitted by this uranium potassium
sulfate and also the radiation was
emitted from this compound even in
absence of sunlight so sunlight was not
playing any role in this so he concluded
from this and then he did a lot of
research on this compound and he found
that it was uranium alone which was
responsible for the emission of this
radiation so this is how radioactivity
was discovered so from there he
discovered that there are certain
elements which emit radiation on its own
and even if you rhenium is kept on its
own you don't give any any specific
experimental set up is not required for
that uranium whenever left on its own
will emit some radiations so this
phenomenon was later named as
radioactivity thank you please visit
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