where v is the frequency of the incident (monochromatic) radiation and P is the
work function, the energy needed to escape the surface. He pointed out that Eq.
19.24 explains Lenard's observation of the light intensity independence of the elec-
tron energy.
Equation 19.24 represents the second coming of h. This equation made very
new and very strong predictions. First, E should vary linearly with v. Second, the
slope of the (E,v) plot is a universal constant, independent of the nature of the
irradiated material. Third, the value of the slope was predicted to be Planck's
constant determined from the radiation law. None of this was known then.
Einstein gave several other applications of his heuristic principle: (1) the fre-
quency of light in photoluminescence cannot exceed the frequency of the incident
light (Stokes's rule) [E5]; (2) in photbionization, the energy of the emitted electron
cannot exceed hv, where v is the incident light frequency [E5];* (3) in 1906, he
discussed the application to the inverse photoeffect (the Volta effect) [E8]; (4) in
1909, he treated the generation of secondary cathode rays by X-rays [Ell]; (5) in
1911, he used the principle to predict the high-frequency limit in Bremsstrahlung
[E12].
7975: Millikan; the Duane-Hunt Limit. In 1909, a second review paper on
the photoeffect appeared [L6]. We learn from it that experiments were in progress
to find the frequency dependence of Em:al but that no definite conclusions could be
drawn as yet. Among the results obtained during the next few years, those of
Arthur Llewellyn Hughes, J. J. Thomson's last student, are of particular interest.
Hughes found a linear E-v relation and a value for the slope parameter that
varied from 4.9 to 5.7 X 10~^27 , depending on the nature of the irradiated material
[H5]. These and other results were critically reviewed in 1913 and technical res-
ervations about Hughes's results were expressed [P10]. However, soon thereafter
Jeans stated in his important survey of the theory of radiation [ J3] that 'there is
almost general agreement' that Eq. 19.24 holds true. Opinions were divided, but
evidently experimentalists were beginning to close in on the Einstein relation.
In the meantime, in his laboratory at the University of Chicago, Millikan had
already been at work on this problem for several years. He used visible light (a
set of lines in the mercury spectrum); various alkali metals served as targets (these
are photosensitive up to about 0.6|tm). On April 24, 1914, and again on April 24,
1915, he reported on the progress of his results at meetings of the American Phys-
ical Society [Ml, M2]. A long paper published in 1916 gives the details of the
*In 1912, Einstein [E10] noted that the heuristic principle could be applied not only to photonion-
ization but also in a quite similar way to photochemical processes.THE LIGHT-QUANTUM 381it reaches the surface. Let £max be the electron energy for the case where this
energy loss is zero. Then, Einstein proposed, we have the relation (in modern
notation)(19.24)