The experimental apparatus was as follows: A white light beam was shined
from a mercury vapor lamp and, passing through a lens/diffraction grating,
spread into a fan of its individual component colors.
The location of the
lens/diffraction grating was adjusted (in the direction of light propagation) to
sharpen the resolution of light on the detector.
The detector could be rotated
to intercept any single maxima of the rainbow diffraction pattern.
addition, the detector window could be masked with up to three different
filters: a yellow filter (i.e., one which allows only yellow light to pass
through), a green filter, and an intensity filter, to block a fraction of
incoming light regardless of color, which itself was subdivided into five
subsections: 20%, 40%, 60%, 80%, and 100% transmission.
measuring a green or yellow maximum, their respective filters were used to
mask ambient room light and overlying higher-order maxima from higher
frequencies from the light source.
Inside the detector, light incident on the photoelectric plate, the anode,
ejected electrons onto the cathode.
Since the maximum kinetic energy of an
ejected electron was limited by the energy, and thus the frequency, of the
light quantum which dislodged it, once sufficient charge had accumulated on
the cathode, the potential difference between anode and cathode would be
too great for a single electron to surpass.
The relationship between this
potential difference (hereafter, termed ‘stopping potential’), the electron
kinetic energy, and the photon energy and frequency will be examined in
more detail later.
The experiment itself consisted of the following measurements: the