In 1895 Heinrich Hertz observed that ultraviolet Light from the sparks of his generator for radio
waves he had recently discovered, falling on the negative electrode of his radio wave detector,
induced the flow of electricity in the gap between the electrodes. Pursuing the phenomenon in
detail, he discovered the photoelectric effect whereby light of sufficiently short wavelength causes
the ejection of electrons from a metal surface. 1905 Nobel Laureate Philipp Lenard
made improved measurements and demonstrated by determination of their charge to
mass ratio that the ejected particles are identical with the electrons that had recently
been discovered by 1906 Nobel Laureate J. J. Thomson in experiments with cathode
rays.
Crude though the early data were, the qualitative fact of the dependence of the critical cutoff
voltage on the wavelength of light emerged with sufficient clarity to induce the young Albert
Einstein, working as a patent examiner in the Swiss Patent Office in 1905, to link the effect with
the recent idea, introduced by Planck in 1900, that matter radiates its energy in quanta of energy
hv. He postulated that light delivers its energy to an absorber in quanta with energy
hv. Thus, if it takes an amount of energy ϕ to lift an electron out of the surface and
away from its image charge, then the residual kinetic energy K of the ejected electron
is
1921 Nobel Prize was awarded to Albert Einstein for his discovery of ”the law of photoelectric
effect”. It was not until 1912 that the technical problems of making precision measurements
of the photoelectric effect were overcome by 1928 Nobel Laureate Sir Owen Wilans
Richardson and K. T. Compton to the point where the Einstein photoelectric equation could
be tested to high accuracy and used in precise determinations of Planck’s Constant
h = 6.626 × 10−34J ⋅ s = 4.135 × 10−15eV ⋅ s
References
This is a derivative work from [1] a Creative Commons Attribution-Noncommercial-Share Alike 3.0
work.
[1] MIT OpenCourseWare, 8.13-14 Experimental Physics I + II , Fall 2007 - Spring
2008.
