Experiment using LED's, voltmeter and a variable power supply (30 minutes)
This is the simplest and cheapest quantum physics experiment around. It can
be made expensive and complicated but it needn't be for a high school class.
It is like the photoelectric effect run backwards.
The theory in its simplest form is that when an electron travels through an LED, it releases one photon of energy. The energy lost by the electron is eV and the energy of the photon is hf. Each coloured LED has a different colour because it has a different voltage drop. The experiment in its simplest form uses the manufacturer's labelling to determine the frequency or else you can match the colour to a colour chart or use a spectrometer. All that remains is to find the minimum voltage that will light the LED. There are two easy ways to do this.
1) In one version, you hook up an LED to a potentiometer (variable resistor) and battery. You measure the lowest voltage across the LED that makes it light up. This technique is described in detail with lots of supporting documents here. This is from the website of the Perimeter Institute for Theoretical Physics. It includes a worksheet for students and extra details for teachers. This technique is great as an experiment for small groups. It requires more time and equipment than the one below.
2) This second version is like a set of Ohm's law experiments. A variable power supply is used instead of batteries and potentiometer. The voltage and current is recorded for each LED and plotted. The V-intercept is used as a measure of the cutoff voltage. Afterwards the analysis is similar to the above. A worksheet for this version can be found in this document and was developed by Alan Linville of Alberta. I like this version, because the students clearly see how non-ohmic an LED is. It can be done as an interactive demo, using overhead volmeters and ammeters or in small groups. It tends to result in smaller errors in h.
NEW! An Even Easier Version of the Experiment
3) Finding and using the potentiometers can be problematic. Most of us don't have lots of variable power supplies. What do you do then? You do this simpler version of the lab that was presented recently by the Alberta Association of Physics Teachers.It doesn’t need a variable power supply - just a couple of batteries to provide an constant 3V. The LED is attached to the batteries in series with a resistor of 10’s to 100’s of ohms. The voltage drop across the LED is measured. This is repeated with several different LED’s but the same voltage and resistor. A graph of the voltage drop vs. the frequency of the light yields a straight line with a slope of h in units of eV.s. What if you don't have the LED's? The LED’s can be ordered online. The Source charges $5 per LED but they are only $0.25 each from this site in Manitoba. Further details about the typical results and a discussion of the strengths of this version compared to the other versions can be found here.