We recently introduced the h/e experiment into our introductory physics
labs, using the Pasco apparatus. Since this was my first time through
the experiment, I was paying close attention to all the details of
procedure, use of filters for yellow and green lines, etc. I noticed
several things, such as that the voltage measured for the yellow and
green lines is lower with the filters, but it doesn't matter which
filter. In other words, you don't have to use yellow with yellow and
green with green. Either filter works fine for both lines. But without
a filter, both lines give voltages that are too high. Obviously there
is interference from somewhere, but where?
In puzzling over that (sometimes you reinvent "the wheel," I know) and
discussing it with another member of our faculty, a plausible
explanation seemed to be that 2nd order uv was interfering with the
yellow and green in first order. The wavelength of yellow (578 nm)
would require a uv line at 289 nm to put its 2nd order at the same angle
as the 1st order yellow. And to overlap the green (546 nm), a uv line
would have to be at 273 nm. So, next day, another faculty member (John
Sohl) and I checked the spectrum with John's computer-based
high-resolution spectrometer that goes down well into the uv to look for
those lines. We could see nothing in the range of 300 nm or down. So,
we began checking the spectrum at the angle of the yellow line. Lo and
behold, there was the entire spectrum of mercury, just at lower
intensity for all lines except yellow. We saw, in particular, the uv,
v, and blue lines.
The best explanation we could come up with is that there is some
diffuse scattering of the entire spectrum into angles where you expect
only the maxima from individual colors. And at the positions of the
yellow and green lines, which are at lower photon energies with lower
stopping potentials, the small contribution from the uv-v-b lines is
adding a small voltage in the op amp circuit. Not enough to look like
uv, v, or b, but enough to give an erroneous reading at the lower
voltage lines. The thing that the filters do is remove the uv, v, and b
lines from getting to the photodetector when you are looking at the
green or yellow lines.
The explanation Pasco gives in the instructions accompanying this
apparatus is that 2nd order uv interferes with the yellow and green.
From our experiments, I would have to say that this explanation is
wrong. We were wondering if anyone else has noticed this effect? Or
has the correct explanation appeared somewhere?
Rondo Jeffery
Weber State University
Ogden, UT 84408-2508
rjeffery@weber.edu