Squarespace the sponsor of this video when you make Metals hot they start
glowing red kind of like coal or lawa if
you make them hotter they start turning
orangish this is an everyday phenomena
so the physics over here must also be
pretty
straightforward or is it in the 1900s
the color of hot glowing things baffled
even the greatest Minds because it
showed us that there was something
fundamentally wrong with physics and
it's what led Max plang to quantum
theory
purely by accident but what is so
baffling about hot glowing things well
when I first learned about this in high
school the way I understood it was
something something black body radiation
something something classical physics
fails ultraviolet catastrophe oh no and
then something something MX plun comes
along proposes e equal to H new and
quantization and it solves everything
and quantum theory is B but when I went
into the college I then started getting
some details in fact I started getting
too much details I only got details of
derivations and math and and graphs and
whatnot and again the big picture was
lost I after all of this I couldn't
answer the fundamental question of why
did hot glowing things break physics and
how did it lead to the quantum theory
what is the intuition behind it so for
the past few days I have been breaking
my head going over a lot of resources
and even reading books for that matter
and I finally think I got some really
really strong intuition and I want to
share that with you so this is the first
of a three-part video series where we're
going to intuitively ReDiscover the
whole thing and in this video we're
going to we're going to see how it all
started what the heck is a black body
what's that got to do with glowing
things and why did it baffle physicists
back then and the next video we'll try
to attack this problem using the known
laws of physics and ReDiscover the whole
Ultra wallet catastrophe and finally we
will see how quantization solves it all
so if you're ready for this let's begin
so we're going to start with the biggest
and the most fundamental question I had
when I was reading this particular topic
and that is why do we care about black
bodies and to answer that question we're
going to talk to the man who started it
all Gustav Robert kof kof why do we care
about black bodies and kof says Mahesh
if you take an apple and you keep it in
the sunlight it gets hot why and I'm
like totally unrelated but that's
because the apple is absorbing all the
sunlight and that energy is getting
converted into thermal energy raising
the temperature and kof is like almost
correct but remember that apple is not
absorbing all of the light some of the
light is getting reflected that's why
you see the Apple but yes the rest of
the light is being absorbed it's
converting into thermal energy and the
temperature is Raising but that apple is
continuously absorbing sunlight and
therefore its thermal energy should
continuously increase and so the
temperature should continuously increase
eventually vaporizing the apple right
but that doesn't happen why not and I'm
like wa that that is a beautiful
question K I never even thought about it
so why doesn't it happen well K says
this could mean only one thing not only
is that Apple absorbing the sunlight
that tends to increase the temperature
it must also be radiating it back that
tends to decrease the temperature and
that's how the temperature stays steady
in other words the apple is in thermal
equilibrium and therefore that Apple
must not just be reflecting light it
must also be radiating light it must be
creating its own light it must be
glowing and he call this the thermal
radiation now kof has no idea what
causes the thermal radiation because
electrons were not discovered back then
but today you and I know thermal energy
is basically jiggling electrons and we
know jiggling charges produces
electromagnetic waves and that's what
causes thermal radiation but kop was
able to argue purely from energy
conservation point of view and that I
find is fascinating it just goes to show
how powerful energy conservation is but
anyways this means K off that apple is
glowing it's producing its own light
right and Kish says not just the Apple
mahes everything around you including
you me and donkeys we are all glowing
but then I'm like if we are all glowing
due to thermal radiation why don't we
see it if I switch off the light why
don't you see me glowing ah says kha
well that's because you're not glowing
in the visible region at very low
temperatures like at room temperature
most of the radiation that we giving out
is in infrared region and that's why our
eyes cannot detect it but if you use an
infrared camera then we can see it today
but now if you could increase the
temperature to thousands of degrees C
then we can see visible light coming out
and that's why you know Metals glow red
hot or cold or lava they start giving
you all of that color and then if you
could increase the temperature even more
to about say 10,000 de C now we'll start
even getting ultraviolet light and
that's why welders for example need
protection from ultraviolet light and so
the color of this th radiation depends
on temperature but whatever it is we're
all glowing as long as you have
temperature we all glowing due to
thermal radiation and I find this
fascinating because you know in earlier
in primary school we had learned about
luminous objects like light bulb and Sun
that give out their own light and non
luminous objects like you and me and
donkeys but turns out we're all luminous
okay all of us are actually giving out
light just that we're not all giving out
invisible Spectrum we're giving out
mostly in the infrared light but we are
all luminous we're all glowing which is
super awesome now this is all great kiss
off but the big question is why does it
behave that way I mean why is it at low
temperature I most gave out just
infrared and then at higher temperature
start giving out visible and then even
UV and kov says exactly mahes that's
exactly what the question I had and
that's why I was urging the scientific
Community to do some research on black
bodies because that way we would get to
the bottom of this but I'm like wait a
second wait a second what has this got
anything to do with black bodies Kira
asks mahes you are now giving out
thermal radiation what does that depend
on and I'm like it depends on
temperature we just talked about it I'm
giving out infrared light
kof says no not just the temperature see
if you're wearing clothes that are
reflective highly reflective of say
infrared then a lot of the infrared
light that you're giving out your body
is giving out will be reflected back and
most of it will not make out so the
thermal radiation that finally comes out
not only depends on the temperature but
also depends upon what you're wearing
how thick it is and all of that but what
if the clothes that you're wearing were
100% absorptive zero Reflections then
all of that infrared will be completely
absorbed and then reemitted and now all
of that infrared will come out and now
it will be independent of how thick that
material is or what that material is
made of as long as it's 0% reflective
100% absorptive all of the thermal
radiation will come out in other words
if you had an object that was 100%
absorptive zero reflection the thermal
radiation that comes out will have
nothing to do with the surface feature
will have nothing to do with the
material is made of it has nothing to do
with anything except for temperature now
the thermal radiation would be
purely a function of temperature in
other words we are staring at a
fundamental law of Nature and I'm like
that's a very interesting way to put it
kop wow but what kind of material would
give you zero reflection 100%
absorption black bodies and Kish says no
no no perfect black bodies it's an
idealized concept a body that will not
reflect anything so even the even the
black objects that you have around you
they do reflect something that's why
they Shine That's why you can make out
the surface feature a perfect black body
would not reflect anything not nothing
invisible nothing in front nothing in
you with nothing at all 100% absorbent
and the th radiation provided produced
by a perfect black body would only
depend on temperature that was the big
deal and that's why kof was urging the
scientific Community to do research on
that that's why hard glowing things was
a very interesting subject so putting it
all together kof loved perfect black
bodies because the thermal radiation
that they would give out would purely
depend on temperature so analyzing it
would be analyzing a fundamental law of
physics so what happened next and kof is
like for the next 20 years nothing
happened and I'm like why not kin says
what do you mean why not mes it's the
1860s we don't have the technology to do
any of these experiments yet but in the
early 1900s we finally started doing
experiments on perfect black bodies and
I'm like wait a second wait a second
what you mean we finally started doing
it perfect black bodies do not exist how
can you do experiments with perfect
black bodies turns out we found an
ingenious way to actually build a
perfect black body it's kind of like an
even save 10% by using the promo code fload physics at the checkout all right
back to hot glowing things again how
exactly do we build a perfect black body
well in from the 1890s to 1900s there
were a ton of people who did a lot of
research both experimentally and
theoretically so let's talk to one of
the prominent ones Auto lummer Mr lummer
how exactly do you build a perfect black
body it's an ideal thing how do you
build one Lumar says using a hole in a
box and I'm like what take a hollow
metallic cube with the walls painted
black and put a hole on one of the face
in one of the faces now that cube is
certainly not a perfect black body
because it's going to reflect a lot of
light but what about that hole if you
shine light into that hole there's a
good chance that light is pretty much
lost inside it'll be absorbed the chance
of that light coming back out of that
hole is almost zero in other words that
hole is pretty much 100% absorptive zero
reflective that hole acts like a perfect
black body and I'm like wait a second
that's cheating that's a hole it's not a
thing and lommer says M remember what
kof taught us it doesn't matter what
that material is made of that is
irrelevant as long as you have something
that is 100% absorptive 0% reflective it
is a perfect black body its thermal
radiation would purely be a function of
temperature that whole is pretty much
100% absorptive 0% reflective so that
whole nothing else just that hole acts
like a perfect black body so if you heat
the whole metal up to very high
temperatures and look at the radiation
that's coming out from the hole not from
the rest of the cube but just from the
hole and analyze it that would be
thermal radiation of a perfect black
body and that's exactly what they did
and I'm like lummer along with your
friends please take a bow because that
is incredible remember we're talking
about the 1900s they found a way to
create a perfect black body and this is
one of the the reasons why I think we
should when we learning physics or any
subject for that matter we should dive
deep into the experimental details
because it's going to give us a lot of
insights and it's going to help us
appreciate the subject even more but the
big question now was what are the
findings of this experiment well the
findings can be summarized by drawing a
graph yes this is the famous black body
radiation graph where on the x-axis
we're going to plot the wavelengths
which you can Loosely think of it as
colors and on the Y AIS we're going to
plot the intensity which you can Loosely
think of it as the brightness of the
different colors that are coming out out
so Lumar what did we find lummer says
first of all at whatever temperature you
choose the thermal radiation will always
contain all the colors of light that
means that right now you me and donkeys
we're not just radiating in infrared we
are radiating invisible we are also
glowing in ultraviolet we're glowing and
giving out all the colors of Light which
is pretty awesome if you think about it
but the reason why you cannot see me is
I mean when in in the dark for example
why don't I glow in the dark you can't
see me is because not all the different
colors have the same brightness they do
not have the same intensity turns out at
room temperature the graph looks like
this you can immediately see the amount
of ultraviolet and the visible that
we're all giving out is negligible it's
almost zero it's there but it's almost
zero but most of the intensity is in the
infrared far right infrared region and
that's why at room temperature we mostly
give out infrared light but what happens
if we increase the temperature well
let's increase it to say about 2500° cus
what is the graph look like it looks
like this you can see the graph is now
so big it goes out of bounds we can't
even see it anymore and that's because
at higher temperatures you have more
thermal energy and so the thermal
radiation is much higher and so all the
colors of light will now get more
intensity but if we scale down the graph
we rescale the graph you can now see
that this time although the graph has
become bigger now the a lot of visible
light is coming out this is because the
peak has shifted to the left that's the
that's one of the key things over here
and this is the reason why when we heat
up Metals For example you now see them
glowing reddish because now visible
light comes out mostly in the red end of
the spectrum and if we increase the
temperature even more the same thing
happens the peak shifts even more left
and now a lot of visible light starts
coming out I mean still you're getting a
lot of infrared that's there but now
almost all the colors of the light
visible light is also coming out and now
the object will look pretty much whed
and that's what happens to the sun the
sun has a temperature of about
55° and that's why it glows white I mean
it looks yellow to us because of the
atmosphere but if you went outside the
atmosphere looked at it the sun is
pretty much white because that's the
spectrum of the Sun and of course if you
increase the temperature even more the
peak shifts to the left even more and
now we get a lot of UV but the ultimate
question is why does the graph look like
this why is it at low temperatures we
mostly get infrared and then as the
temperature increases the peak start
shifting to the left and you start
getting more and more VIs visible in UV
that was something that people were
trying to figure out and in order to do
that people studied the graphs even more
and they came up with some results some
very interesting results the first one
is the Stefan bolman's law we call it
they figured out that the area under
this graph which is basically the total
intensity given out by all the colors of
light at a particular temperature is
proportional to the fourth power of that
temperature this is the reason why when
we increase the temperature the graphs
blows up and scales up so quickly
because a little increase in the
temperature will give out a lot more
output power another brilliant analysis
was done by a scientist named William
Wan he figured out that the vavin that
corresponds to the peak intensity it's
inversely proportional to the absolute
temperature again that kind of makes
sense when you look at the graph as the
temperature increases as we saw the peak
shifts to the left so that's exactly
what they're saying but he figured out
that it was actually inversely
proportional to the temperature and he
also figured out what that
proportionality constant is now these
two laws do not explain why the graph is
the way it is but I find them
fascinating because we can do some
pretty cool things with that for example
by using the wi displacement law I can
now look at the temperature at the
Spectrum of the sun figure out what the
peak wavelength is and from that I can
figure out what the temperature of the
Sun is that's how we know today that the
sun's surface temperature is about 5,500
de C how cool is that and not just that
now that now that I know the temperature
of the Sun I can use use the stepen
bolman's law to figure out what is the
total energy that the sun is giving out
and then I can figure out how much of
that energy Earth is receiving and then
if I build a solar panel I can then
estimate how much electricity I can
produce just by using solar power I mean
this is just mindboggling but here's
what's really mind-blowing guess what's
the most perfect black body that we have
access to today just just
guess well it's the cosmic microwave
background yes the remnants of the big
bang turns out it perfectly matches the
black body radiation curve with its peak
wavelength lying in the microwave region
and again I can use the v's displacement
law to figure out what this temperature
is the temperature turns out to be
insanely low so black body radiation
isn't just a theoretical concept it's
the key to unlocking the deepest
Mysteries of the universe it tells us
that everything is glowing it helps us
figure out the temperature of the stars
and it allows us to analyze the oldest
light from The Big Bang itself
but but the ultimate question is why
does the graph look like this when we
try to attack this problem using
classical physics we ran into a
catastrophe which we'll explore in a
video here and then we will see how
quantization saved the day in the video
over here I'll see you