Quantum physics gets it’s name from the quantization of particle energies. In quantum physics particles can only have very specific values for the energy, and they must all be multiples of ћ (h-bar) However, in every-day life these energy levels are so close together that you don’t really notice the difference. On top of that we are told that the color of light relates to this value for the energy and the way that light interacts with the allowed energy levels of electrons in a material is what makes it clear, or opaque or what makes it glow. Despite the coherence of this picture, few of us have taken the time to test these concepts first hand. In this second of my “Quantum Basics” videos I tackle an experiment that will allow you to test that light really does have quantized (specified amounts of) energy per photon.
Materials: The main item you will need for this experiment will be a multimeter or voltmeter. All multimeters have a voltage setting and it’s pretty hard to find a voltmeter by itself. They can either have an auto select range or a manual one, either work for this experiment. They are easy to order online, but multimeters can also be found in most big box and hardware stores, usually shelved with automotive equipment. You will also need two different colored LEDs (light emitting diode). Make sure when you order them that they have clear glass since a colored glass cover can interfere with the measurement of the voltage later. Also make sure the LEDs aren’t too close in color, if you have an orange and a red the energy difference may not be enough to see what we’re after. Lastly you will need some way to light up the LEDs, a few 3volt coin batteries should work fine.
- First select at least two LEDs of each color (total 4). It’s also a good Idea to mark the side so you can keep track of the color while performing the experiment.
- Turn on your multimeter and set it to the voltage measurement setting.
- Hook up one of the lower energy LEDs you have (If you’re not sure which is lower use ROYGBIV red is low and blue or violet is high). Depending on the kind of leads on your multimeter you may need to fiddle around with this. Gator clips work the best but if you have point leads it’s simple enough to wrap the tails of the LED around these.
- Depending on the lighting in the room and the kind of LED you have you may get a small voltage reading right away. If you get a negative value it just means the polarity of the LED is reversed from how the multimeter is measuring it, it doesn’t really matter in this case since you’re just looking for size of voltage not polarity.
- Next pick up the other LED of the same color as the one currently attached. Hold the ends against either side of the coin battery, if nothing happens try flipping the LED around.
- Shine this – Same Color – LED (red in my case) into the one hooked up to the multimeter. The glass cover will act like a little lense so they won’t get very good results if you shine from the side, but if you angle the lit LED to point downward into the other one you should see a sudden jump in the voltage reading on the Multimeter (If you’re feeling ambitious record the voltage).
- Next shine your other color light (green in my case) onto the first LED. Again you should see a signal (record the voltage).
- Repeat steps 5-7 but swapping the colors. In my case this menant attaching the green LED to my multimeter and shining first red then green lights on it.
- With your high energy light attached to the multimeter (green) try to measure a signal by shining 2 or more low energy LEDs (red) to see if any number of them can overcome the energy gap in the LED being measured.
Explanation: This experiment is meant to explore the idea of quantized, or specific sized, energy levels in quantum systems. Normally this subject is introduced in the Physics Classroom via the photoelectric effect. This is similar in that the photoelectric and photovoltaic effect both pertain to light striking an electron and moving it away from it’s host atom. However in the classic demonstration of the photoelectric effect the demonstration must be done with ultraviolet light, because the kind of interaction requires the electron to leave it’s host atom completely and thus takes a lot more energy. Similarly the photoelectric effect is harder to measure since the energy from the ejected electrons is not easily measured. The photovoltaic effect on the other hand can be low energy since the electron only needs enough energy to move gradually from atom to atom. Also the photovoltaic effect is more prevalent in our modern lives with, light emitting diodes and solar cells. Finally since solar cells and LEDs are already manufactured for electrical devices they are easy to adapt to measurement and can be done with off the shelf supplies.
In the specific case demonstrated the diode within the LED was made of two types of crystal, P-type and N-type.
Each crystal is nearly identical to each other, in fact most of the crystal is made of the same stuff, but in one crystal a small percentage of the atoms has been swapped out for different ones, in particular different ones with one less proton and one less electron. This is called the p-type and creates little energy wells, or zones near these atoms where electrons can settle into. Note that the crystal is still neutral, it’s just that since so few of the atoms were replaced these doping imposters have no choice but to fit into the mesh. Likewise with the n-type crystal, but in this type an atom with an extra proton and an extra electron are doped in which forces the electrons into higher energy packing around these atoms.
Now when these two types of crystal are joined together the high energy packing of the n-type crystal pushes some of it’s electrons into the low energy packing of the p-type crystal. Even though just the electrons are moving it looks like the positive and negative charges swap places and create a static charge at the boundary between the two layers.
Now it is important to note that the light does not come from this electrical charge. what actually creates the light in the LED is new electrons settling down into the low energy arrangement of the P-type crystal. In fact when two crystals of these types are first brought together there would be a brief flash as the electrons flowed from one arrangement to the other. This is EXACTLY like electrons jumping from one orbital to another and emitting light. The only difference is that instead of moving up and down orbitals of one atom, they are moving up and down bands of the combined orbitals of all the atoms in the crystal. In fact these kinds of sudden rearrangements in electrons can be caused by stress in the material as in triboluminescence.
To repeat, it is not the charge distribution at the boundary that releases the light, the static charge simply keeps electrons from flowing indefinitely, and can give valence electrons a push in the right direction if they get excited by light falling onto the crystal. In this experiment this is exactly what happens, when light with enough energy hits an electron near the boundary it can get pushed back into the high energy packing of the p-type crystal at which point the static charge from the other electrons still at the boundary pushes it out of the crystal. It is this motion that creates the voltage we measure. This is also the way that silicate crystal solar cells work to generate electricity.
Simplified Results: I didn’t record my exact voltage values this time, mostly because it was difficult to control for the angle of the LED and the ambient light. However the main point of this experiment was simply to show that some colors of light do not have enough energy to move an electron over the charge barrier in the LED no matter how much light there is, and yet a very small amount of light of the other colors can create a voltage.
In my case I was unable to get the green led to show any response while shining red light onto it. I demonstrate 5 in the video, plus the toy laser. In additional tests I tried as many as 10 still with no signal. This shows that it is not the amount of light that hits the LED that determines the energy received by an electron, but the character of that light. Only light of a wavelength equal to or greater than the LED being tested was able to make a signal on the voltmeter.
Make magazine collin’s lab inspiration for this episode.
MIT OCW Photovoltaics – within playlist
Veritasium Transistor explination