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And in 1900,
P. Leonard he was looking at the photoelectric effect
The photoelectric effect is when you send say some lights in this case UV lights
and if it hits the surface of the metal
you liberate an electron, so an electron will fly out
now he wants to measure the energy
that these electrons have as they come out, so what he does is, in this evacuated chamber, so that's a vacuum inside
he sets up a potential difference, so he builds up all kinds of negative charge on this side
and a bunch of positive charge on the other side
and so as electrons fly out with a certain initial speed given to it by the light
it is eventually going to turn back
some are going to turn back, some are going to make it, and the one that makes it will be registered
as a current over here
so he wanted to see if he change the intensity of the light to give it more energy
is he going to get more energetic electron or whichever
so in terms of the math, we have
...
...
...
delta V here is
the stopping potential, it is the
potential that is needed to stop all the electrons from reaching the other plate
so that is telling what is the maximum kinetic energy the electron flies off at
but what was weird was, if we want to increase the energy of the light, we would increase the amplitude, because recall
that the intensity (I won't use "i" again, because it is confusing with current) is proportional to A^2
now you'd expect more energy means
electrons fly out faster
and the stopping potential would increase, but what he saw was that
as you up the intensity
that current increase but then
the delta V stays the same
weird enough, delta V actually depends on the frequency instead
that was kind of weird, why is it that when you change the frequency of the light
the electron absorb more energy
whereas if you just have more light
you just get more electrons but not more energetic electrons, so that was really bizarre
not only that
because we envision the UV light as a wave, you'd expect that it takes some time for the wave
to travel through and into metal before you build up enough energy to dump an electron
but that didn't happen, as soon as you turn on the light, it is possible that
right away an electron would fly out
so this really
upsets the view of lights being a wave
as you don't expect a wave to be absorbed all at once
and you don't expect the energy of the wave to depend of its frequency and not its amplitude
so that jimmy things up a little bit, and it is not until 1905 when Einstein took Planck's idea that we can start
to understand how this would lead to the quantization of light, but let's go through Planck's idea first
so at the same time, Planck as well as other physicists were
struggling to explain
these things called "blackbody radiation"
by the way, I will be introducing a lot of names and various famous experiments, but don't get too caught up in
franticly taking notes, just sit back and enjoy the progression of the ideas
so what the heck is blackbody radiation
first, let's talk about thermal radiation, so you might remember that
if you have a body at a certain temperature, it's going to radiate out
some kind of radiation, infrared rays most likely for room temperature
but as you get hotter, it is going to glow into the visible
so similar to this piece of metal here, as you get hot, it's going to get into your red, then your yellow and so on and so forth
if you get bright enough, it would turn white glow then eventually blue glow
so thermal radiation, we expect, as the temperature gets hotter
the peak frequency would shift
now to try and explain this phenomenon, we have an idealized case
for real object, you have things like reflection involved
and they are not perfect emitter or absorb all the light [from outside sources]
what we want for a blackbody is an ideal case where
it's a perfect absorber and emitter, because
these two processes are exactly opposite, so if you are good at one then you are good at the other
so that's why you call them a blackbody; they are really good at absorbing stuff
and at the same time, they are really good at emitting their radiation as well
as kind of an idealization [to remove interaction with the outside], physicist made up this kind of idea
to avoid any external
influences and to get all the radiation from it, what they do is they set up a closed vessel like this and punch a hole on the side
everywhere on the inside you have the temperature T, and these thermal radiation goes and bounce back and forth
all different ways
and so and so forth randomly, until a little bit of it escapes out this side, then they have a detector over here that detects everything
now
when they try do the calculations for it, they postulate that if we have a vessel, we should have standing waves inside
and because it is very big [with respect to the wavelength], you can lots of modes that's possible
how much energy as in each mode
we can get from statistical mechanics which can tell you how much of these
particles on the edge
what energy should they be having, and therefore how much they should be emitting
now when they did the calculation for it though, what they got was
as lambda gets small, or frequency gets large, the amount of energy in these modes
grows expontentially, no, not quite, but a harsh power function
but it goes towards infinity, which is completely nonsense because
it is easy to observe the energy gets a peak frequency and then levels off after that
so Planck came up with this idea that, in order to explain this,
that all these particle that is doing the emitting of the EM waves
they themselves are compound harmonic oscillators
and therefore they have modes as well
these harmonic oscillators have
certain energy levels it is allowed to be in, it can resonate in certain different modes
and then he is saying that these modes have energy of some integer multiple of a constant times the frequency
that are shaking at, which is also the frequency of light end up emitting
we use "nu" for frequency instead of f
and "h" here is the Planck's constant
...
and when he punched that in to limit the number of modes at the higher frequency
we got a result much much closer to the actual observed spectrum
now we are still unsure where this comes from
but Einstein took it one step further