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⁾

hν = W + E_kin

¹) E. M. Rogers, Fizyka dla dociekliwych, PWN Warszawa, 1972
²) who, besides, got the Nobel Prize for this