Ships in the harbour

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 (=E_kin , i.e. its velocity). The reasoning so simple, that an Einstein was needed to do this²⁾

1) E. M. Rogers, Fizyka dla dociekliwych, PWN Warszawa, 1972

2) who, besides, got the Nobel Prize for this