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Chapter 3.1b Blackbody Radiation Ultraviolet Catastrophe
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the topic of this video is
electromagnetic energy and specifically
black body radiation in the ultraviolet catastrophe
catastrophe
in addition to the learning objective
that is written on the screen um i just
want to provide some context what we're doing
doing
here is building up a little bit of
background before we jump into quantum theory
theory
of the atom so we are going ultimately
building up an
electronic structure uh model of
the atom but first we need to consider
some some
uh historical developments so black body radiation
radiation
in case you are unfamiliar or it's been
a while since you've
you've heard of this term it is the thermal
thermal
electro magnetic radiation within
or surrounding a body
object alternatively so don't be
confused by body it's not
an actual you know body of an organism it's
it's
it's an object okay and um what happens
is if you
take an object and heat it up think
about uh
a piece of of metal like iron a steel bar
bar
if it gets really really hot it's going
to start glowing it's going to start
emitting electromagnetic radiation
and that's going to be a function of its temperature
temperature
so one example of a black body radiator
is actually
within our own solar system it's our sun
and so what i'm showing here is a plot
of spectral irradiance so sort of the
intensity of the
of of light as a function of the
wavelength of the light being emitted
so the golden color uh these are
actually look like tc and j colors this
this golden yellow and the blue
um the the you know really sort of
rough looking yellow one is the actual uh
uh
light being measured being emitted from
the sun
and then the blue line in this plot is of
of
sort of a well-controlled experimental
black body
radiator something like an oven that is
being controlled the temperature's being controlled
controlled
very carefully and the light being
emitted is being monitored
very accurately and you can see that
actually the the
uh the solar spectrum of light emitted
from the sun
is is uh quite similar to that of um
sort of a model black body radiator at about
about
over 5000 degrees celsius
i mentioned that it's as a function of
temperature so if we look here this is
again intensity
on the y-axis and then we are looking at wavelength
wavelength
on the x-axis and the point of this is
just to show you that
if you take a black body radiator and
you start at some a temperature such as
2500 kelvin
um it will be emitting sort of this red
light but as you increase the temperature
temperature
uh it will not only emit more intense
light but that light will
shift into a different part of the the
spectrum and become more
visible and you also get uh more of the
or all of the visible um spectrum colors
and that's one of the reasons why a very
very very hot object
would appear white to us we see all of
the visible light
being emitted whereas a hot but maybe not
not
quite white hot object would be glowing red
red
okay so um where what is
what does this have to do with the
ultraviolet catastrophe what is the
ultraviolet catastrophe so in the early uh
uh
1900s physicists were trying to
model black body radiation and uh
so the the plot here is again radiation
density or just sort of you know
uh uh spectral uh
uh intensity as a function of wavelength
so very similar to the plots already
looked at
now the physicists had built a model that
that
worked fairly well um when you look in this
this
range over here sort of the longer
wavelength ranges you can see that the
red line and the black line for the for
the model and the observation were
very good but the catastrophe has actually
actually
uh over here i'll highlight this area in green
green
where you can see the observation dips
down uh
as as the wavelength is going down the
spectral intensity
also goes down but the models in the
early 1900s actually predicted the
opposite it predicted that
there would be almost infinitely more light
light
being emitted as at lower wavelengths
which is definitely not the case
so this was a a a pretty big
issue because the model was very clearly
not describing reality
that well okay so uh
what happened was this german physicist named
named
uh max planck
um came along and he had the idea to
what he thought
was a gimmick he said he thought well
you know rather than allowing
uh all types of vibrational frequencies
to be considered for a black body radiator
radiator
what if we consider them to be discrete
or quantized okay
so and think about quantization like i
discussed for standing waves in the
previous video
so what he ultimately decided to to
suggest is that you model the energy of
a blackbody radiator as e
equals n h nu
and what makes this quantize is that n
is a variable that
has to be a whole integer value one two three
three
example uh uh and then
as examples but it can continue as any
whole integer
up to infinity okay um and it's unitless
it's just it's it's
it's a quant it's a it's a quantum okay
it's just a
exact number um the h value here
is what we now refer to as planck's constant
constant
and that has a value of 6.626 times 10
to the negative 34.
joule seconds it's joules in seconds
multiplied by each other
and then um nu over here is just the
frequency still so that's still
hertz or one one over seconds so this
was kind of a revolutionary idea
um uh at first he thought it was just a
way to get it to reconcile
the model which it did it actually
allowed the model to accurately predict
black body radiation at all wavelengths um
um
but what it turns out uh max planck did
is he sort of turned the field of physics
physics
to a quantized view of matter
and energy levels within matter which
was a
major development within um uh all of
science really not just physics it was a
had a
uh phenomenal impact and and sort of
deepened our understanding of the
electronic structure of
matter and ultimately led to uh the
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