This will be a good bridge between optics and our last major topic, modern physics. This video will go over a lot of introductory modern physics topics such as blackbody radiation, the photoelectric effect, and ideas of quantization. As you go through this video youll not only learn about more properties of light, but also very important physics ideas and phenomena that are causations for some of the most advanced technology to date. This topic has a low chance of being on the exam in quantity, but its still quite a useful topic, and a great one to close optics and open modern physics, the final chapter.
Light (and all EM radiation) exhibits characteristics of both waves and particles.
Evidence for the wave nature of light includes diffraction, interference, Doppler Effect, and Young's Double-Slit Experiment. Evidence for the particle nature of light includes Blackbody Radiation, the Photoelectric Effect, and the Compton Effect.
EM radiation exists in discrete amounts, known as photons. Photons have zero mass and zero charge, but they do have momentum. The energy of a photon is directly related to its frequency by E=hf.
When light is incident upon a metal, electrons may be emitted, known as photoelectrons. Photoelectrons are only emitted if the photons incident upon the metal have an energy greater than the metal's work function, which corresponds to the energy binding the electron to the metal. Any excess energy becomes the kinetic energy of the photoelectron.
The Compton Effect showed that photons of light also have momentum.
If EM waves can behave as moving particles, moving particles can behave like waves. This was shown in the Davisson-Germer experiment, when electrons shot through a double slit produced an interference pattern.
The wavelength of a moving particle is known as its de Broglie Wavelength.
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.
Hi everyone. I am Dan Fullerton and I would like to welcome you back to Educator.com. 0000
Today we are going to start talking about modern physics topics, specifically wave particle duality. 0004
Our objectives are going to be to explain what is meant by the dual nature of light, to describe evidence indicating both the wave nature and the particle nature of electromagnetic waves, determine the kinetic energy of an emitted photoelectron giving the metals work function and the frequency of incoming electromagnetic radiation...0011
...to calculate the de Broglie wavelength of a moving particle, and explain the significance of the Compton Effect. 0028
So talking about the duality of light -- electromagnetic radiation exhibits characteristics and properties of both waves and particles. 0037
Particles of electromagnetic radiation are known as photons, a good vocabulary word. 0046
Electromagnetic radiation, therefore, has a dual nature, the nature of wave and particles. 0053
Some evidence that it behaves like a wave -- It diffracts, it interferes, the Doppler Effect, and Young's Double Slit experiment, showing interference again. 0060
Our particle evidence is blackbody radiation, photoelectric effect, and the Compton Effect.0070
That is what we are going to focus on here today. 0076
Blackbody radiation and what is called the UV catastrophe -- radiation emitted from a very hot object known as blackbody radiation did not align with physicist's expectations or their understanding of light as a wave. 0080
Very hot objects emitted radiation in a specific spectrum of frequencies and intensities. 0094
Hotter objects had higher intensities at lower wavelengths.0099
Cooler objects emitted more intensity at higher wavelengths. 0103
Now physicists expected that at very short wavelengths, the energy radiated would become very large and that did not occur and that was known as the UV catastrophe; they did not understand it. 0107
What they expected based on classical theory was a graph of wavelength versus intensity that kind of followed that shape. 0118
That was their classical expectation, instead, what they found was that it tailed off here. 0125
Max Planck proposed that atoms could only absorb or emit radiation in specific non-continuous amounts known as quanta, which solved this puzzle and earned him the Nobel Prize in Physics in 1918. 0135
So he was able to explain this, and without going into a lot of depth, that was one piece of evidence that light, electromagnetic radiation, has a particle nature. 0146
Now the photoelectric effect is another big one. 0158
Electromagnetic radiation striking a piece of metal may emit electrons, which are known as photoelectrons. 0161
Not all electromagnetic radiation created photoelectrons though.0169
Each metal has a certain threshold frequency of incoming light below which no photoelectrons were emitted. 0172
You had to have a high enough frequency for a specific metal for photoelectrons to come out. 0179
If your frequency was lower than that, it did not work and that frequency depended on what metal you were shooting the light at. 0184
Increasing the intensity or the brightness of the light did not increase the kinetic energy of any of the emitted electrons. 0192
Albert Einstein did some work to explain this phenomenon in 1905. 0199
He said "If energy exists in specific, discreet amounts, then electromagnetic radiations exist in specific, discreet amounts," known as photons. 0204
A photon has 0 mass and 0 charge and its energy is quanta, as it comes only in specific amounts. 0212
The energy of a photon is directly related to its frequency where the energy of a photon is equal to (H), Planck's constant times the frequency or since we know frequency is going to be C/λ, that is HC/λ for an electromagnetic wave. 0220
(H), Planck's constant is 6.63 × 10-34 Joule/seconds (J/s) and if you were to graph this, the energy of a photon versus its frequency, the slope here will give you exactly Planck's constant. 0237
Imagine the electrons and metals are held in energy wells, they are stuck there. 0259
The electrons had to absorb at least enough energy to pull the electron out of the well, so that it could be emitted as a photoelectron. 0264
They could only be released when they absorbed a single photon that had at least as much energy as was required to get out of the well. 0271
The energy required to get out of the well, is known as the work function or phi of the metal and different metals have different work functions. 0279
If you could not absorb a photon that would not give you enough energy to jump out of the well. 0289
The frequency corresponding to the photon that had enough energy to get the electron out of the well was known as the cut-off frequency and if you absorbed a photon that had more energy than was required to get it out of the well, well, any of that excess energy became kinetic energy of your emitted photoelectron. 0294
The electron in the metal absorbs a photon with energy -- E = hf -- greater than the metals work function phi. 0314
So then the electron is emitted as a photoelectron. 0324
Any absorbed energy beyond that of the work function, beyond what was required to free it from the well, became the kinetic energy of the photon. 0327
So the kinetic energy of the emitted electron is equal to the energy of the absorbed photon minus the work function -- anything you had left over. 0335
Kind of like -- imagine you are in debtor's prison. 0346
If you are going to absorb a check from somebody to get out of prison, you can only take that check if it is enough to get you completely out of debt and if you have any excess left over after you pay off your debt, that is your kinetic energy, that is your extra energy to move around, your extra freedom. 0349
Now a typical graph that shows this, shows frequency of the incoming photon on one axis and the kinetic energy of the photoelectrons on the other axis. 0365
The work function of the metal is down here and until you have a photon with enough frequency, enough energy to free the electron from the well, nothing happens, but once you get to that cut-off frequency, any excess energy caused by having a higher frequency becomes the kinetic energy of the photoelectron. 0374
Let us take a look at how the photoelectric effect might work in a circuit. 0397
Let us assume that we are going to take an inner vacuum -- there is our vacuum chamber and we are going to put some electrodes with a metal on them. 0401
We will put one electrode here and we will put a metal on it over here -- make that a little thicker to show that. 0412
We will hook this up running it outside into an ammeter and then bring that over to a source of potential difference and hook up the other side. 0418
We have this (V), positive and negative -- and what we are going to do is we are going to shine a light on our metal. 0429
There is our flashlight -- shining photons on our metal. 0436
Now if those photons have enough energy to overcome the work function to free the electrons from the well, if they are absorbed, then we are going to have the electron get emitted. 0443
Any excess energy of the electron, anything that was not spent in freeing the electron from the well, becomes the electron's kinetic energy and we have a positive voltage over here, to help collect the electron, so that is sometimes called the 'collector' here. 0454
Now, if you reverse this and you make the voltage here too large, then the electron cannot jump across anymore and that is going to happen when you have the maximum kinetic energy of the emitted photoelectron is going to equal the energy that you have here, (Q) times that voltage. 0469
Therefore they call the voltage where this happens -- kinetic energy max divided by the charge -- they are going to call that the stopping potential.0485
At that point, you cannot collect any of those electrons; the voltage is off-setting whatever kinetic energy you happen to have there. 0496
But that is what the photoelectric effect typically looks like in a circuit. 0504
Now in 1922, US physicist, Arthur Compton, shot an x-ray photon at a graphite target to observe the collision between the photon and the electrons of the graphite atom. 0509
The x-ray was scattered and it was emitted with a longer wavelength. 0523
If it had a longer wavelength it must have lost some energy and it lost some momentum, but the energy and momentum lost by that x-ray, a piece of electromagnetic radiation was exactly equal to the energy and momentum gained by that photoelectron. 0528
We are talking about conservation of momentum, while we are dealing with photons. 0542
He proposed this wavelength, the Compton wavelength, equal to the mass of the electron times the speed of light, which is 2.4 × 10-12 m. 0549
The maximum wavelength shift that they saw on the x-ray was twice this constant, it could be anywhere between 0 and twice this value, so the Compton wavelength is half the maximum wavelength shift of the x-ray following this event. 0557
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