Photovoltaic phenomena
-a loss of electricity from charged objects
if illuminated with an ultaviolet light was not so strange before precise
Lenard's measurements (1902):
the emission of electrons (its very occuring or not) does not depend on
the intensity of the light but on its wavelength
.
E.M. Rogers in the book
„Physics for inquiring minds”1)
writes: -„Situation is similar to ships in the harbour. From time to time
one of the ships is ejected up for a hundred meters while the rest peacefully
rolls on the water surface".
In fact, it he case of photoelectical effect, the waves (light) even with a quite big amplitude, but with a too big length are not able to cause the electron emission; light with small length (ultraviolet) does cause the electron emmision: shorter the wavelength - bigger is the energy of the emmited electron.
The explanation, for a modern reader is trivial: shorter the wavelength - bigger is the energy of the light quant. Energy of that quant (=hν, where ν is the frequency of the light) is used to "kick out" the electron from the potential well (with a depth W called a "work function") in which it is trapped. The surplus of the energy is converted into a kinetic energy of the electron (=Ekin , i.e. its velocity). The reasoning so simple, that an Einstein was needed to do this2)