Hey guys,
I've been trying to teach myself Ideas to Implementation because my teacher did a shoddy job and I'm up to the set of dotpoints about black bodies. I can see why the classical ideas about the BB radiation curve didn't make sense but what I don't get is why and how Planck's suggestion that energy occurs in quanta solves this issue. Like what about the fact that energy occurs in packets rather than continuously explains why the graph peaks and falls?
Do we even have to know this?
Also how does the photoelectric effect occur? I've read some sources which say the EMR which strikes an atom causes it to oscillate and if it's charged then the movement of this charged atom releases EMR. I've read other sources which say photons in the EMR strike an electron in the atom and cause it to jump a band, and then this electron falls and releases EMR in doing so.
Hey! Just before I answer, shout out to
a whole bunch of short guides I wrote on the course last year. They might act as good little summaries for your self teaching
So a few things for the Black Body Curve. The law that predicted the theoretical shape was called
Rayleigh Jeans Law, but you don't need to know that law or WHY it predicted that specific curve. Just know that it did, and why it was an issue (the issue being even beyond not matching observation, that energy can't approach infinity for high frequencies, that makes no sense).
Putting energy in packets, with the energy per packet related to frequency, solves the ultraviolet catastrophe. Think of it like this. A BB releases a quanta due to some change inside the BB. An electron might fall down a band and release the lost energy as EMR, for example. So, the frequency of the emitted photon is directly related to the energy change that occurred in the BB, by \(E=hf\). Now, for a super high frequency photon of EMR, we need a huge energy change all in one go.
This is rare. This explains the shape of the curve - At high frequencies, you need a huge energy change in the BB, and these just aren't as common as the smaller energy changes that characterise the big peak in the middle of the curve. The peak of the graph purely represents the frequency corresponding to the most common energy change in a BB of that temperature - This is called the
characteristic wavelength. Basically, we get more intensity in the middle, because it is far more likely that an emitted photon falls in that range. More photons, more intensity - And that's where the graph comes from
This is a tough concept - Happy to explain again if you need!
Photoelectric is a little simpler than your sources make it sound (at least for the HSC level)
A photon of light strikes an electron and gives its energy to that electron. If the extra energy is enough to break the electrons bonds with the atom, it escapes, with a kinetic energy equal to whatever is leftover from the energy of the photon. We can express this:
Here, \(\Phi\) is the
work function of the specific metal, the amount of energy required to free an electron in that metal
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