Too late, I suppose, that I now read this. Not that I'd have what you wanted.
You mention the photoelectric effect and gold leaf and so on. Why do you imagine that is required, though? Consider a vacuum tube diode. You may consider the idea of applying some heat to generate thermally agitated electrons near the cathode, but think about keeping the temperature at ambient for a moment before worrying about that. In the vacuum tube, there are very few free electrons. You place a voltage between cathode and anode to encourage any that exist to move towards the anode. No noticeable current flows. Now, flood the cathode with an intense UV light -- say, from a mercury vapor lamp which is very cheap to buy (do NOT allow that light to reach you; use it only in an enclosed volume) -- and observe the sudden flow of current due to the UV photons exceeding the metal's work function. (I doubt much shorter wavelengths will be present, as the 254nm light is already harder to pass through glass and much shorter wavelengths will be quite reduced by any intervening glass.) Now use an argon light source, let it warm up a bit, and filter it through heavy glass to ensure no UV passes (some argon lamps are really mercury-argon so watch out for that) in and, if you can provide some colored glass to pass through to further ensure shorter wavelengths are removed from the source. If you limit the wavelength of the photons so that they are below the work function of the cathode's metal, you should see no measurable current flowing. LEDs might be another choice for light sources, though they are wide-band emitters. The nice thing about them is that a red LED definitely will NOT emit any UV. Lasers might be another choice, if the school has them.
It might be interesting for you to consider also the idea of using the heater on the vacuum tube to adjust the base level of current due to thermal agitation and, if you can arrange things so that you can select the light's wavelength precisely (grating, beam splitter, thin film filters, tunable lasers, etc) then you might see how adding thermal agitation is convolved with the work function of the metal and the wavelengths you use. Might be some interesting hypotheses to consider, there. Also, think about ways to measure and standardize the power of your applied light. How might you create a beam that directly strikes the cathode target with a known energy per unit time level? In other words, can you set things up so that the UV beam's power is the same as the red beam's and hits approximately similar areas on the cathode? (Is it important to worry about the area impacted or the beam power?) What metals have appropriate work functions for near optical wavelengths such as say 254nm and 670nm light to test the idea? Can these metals be found in some cheap vacuum tube? Are cathodes better or worse than anodes, for this purpose (should you perhaps reverse the connections and apply the negative side of the voltage to the anode instead, because the anode is made of a more optimal metal than the cathode, for example?)
I'd suggest to you that you don't need gold, here, and that there are relatively cheap ways to test this.
Also, you could drop all that and instead try and compute Planck's constant as the slope of a line you develop by observing different colored LEDs with very simple and cheap experimental equipment that anyone can readily afford. Different, but certainly related.
Jon
P.S. Note that the work function of a lot of metals is too high for most visible light to be much use. Sodium and potassium have pretty low work functions (still well above that of many red LEDs, though), but they are easily oxidized metals and not often found in vacuum tubes as electrodes. This is why mercury vapor lamps are often used, since the 254nm line of mercury provides almost 4.9 eV per photon, which is high enough to actually work with some common metals like iron or aluminum. You mentioned gold and I just checked its work function, which is about 5.1 eV. That's too high even for that mercury vapor line, so you'd be pressed to find a bright source for the <243nm light you'd need with gold.
07 Jan 2008, 6:52 pm
