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This one had me puzzled for about 30 minutes. Among the three of us thinking about it, we generated three different explanations. I’m curious about many things:

• What you think we expected to happen and why we thought that
• How you are making sense of what actually happens in the simulation,
• And your thoughts about the two ways that students might have made sense of it

Setup: In the PhET simulation you can link to below, set intensity of the light to some low or mid-level. Next, adjust the wavelength such that a small amount of current is registering.

Question: What should happen to the current as you drive the wavelength smaller and smaller?

Explore: What actually happens? Is this similar or different from what you expected? If different, how are you making sense of what happens?

 Click to Run

Further Questions (in white):

• In this experimental setup, why is there a wavelength that corresponds to a maximum current?
• What parameters is the maximum current setting dependent on? Why these parameters?
• What could you change about the experimental setup such that the current increases monotonically as wavelengths get smaller? (Does PhET simulation allow you to do this?)
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4 Comments leave one →
1. Andy "SuperFly" Rundquist permalink
April 25, 2012 1:39 pm

Cool question. My prediction was that decreasing the wavelength while holding the intensity constant would reduce the number of photons per second and hence the number of electrons per second (and hence the current). That didn’t happen, though it does happen if you lower the wavelength enough.

I thought about it for a while and was given a nudge by the “show only highest energy electrons” button. I realized that as you lower the wavelength, each photon (though there are less of them) can excite deeper electrons. That’s cool!

2. April 25, 2012 9:56 pm

I would expect the current flow to be highest when we have photons that match one of the absorption spectral lines of the target. Indeed, for the zinc target there is a nice peak at 149nm (around 8.7 keV). The interesting thing is how many electrons we get from photons that are not as strongly absorbed.

Of course, I wonder about precisely how the modeling is done, and whether they have accurately modeled all the quantum effects. It would be much better to do this experiment in the real world.

3. secretseasons permalink
April 26, 2012 11:31 am

The activphysics simulation behaves a little differently. Which one is showing the “real” physics?

ActivPhysics gives the intensity in real units (mW) while PHET leaves me wondering what exactly they mean by “intensity” as a percentage. Hmm.

• secretseasons permalink
April 26, 2012 11:34 am

Saying “differently” was a bit strong, as I can’t access the same range of wavelengths, but anyway, you don’t see the same peak in current on activphysics — either because of different physics or because you can’t get those short wavelengths.