Lesson 6 of 11 · 6 min
Where the wave picture fails
NCERT §11.5
Kabir's friend Mira argues: light is a wave, so a bright enough beam of any colour should, sooner or later, shake electrons out. The experiments say otherwise. Why?
The lesson in notes
In short
By the end of the nineteenth century the wave theory explained interference, diffraction and polarisation well. In it, light is an electromagnetic wave whose energy is spread continuously over the whole region the wave fills.
On the wave picture, electrons at the surface soak up energy continuously. Brighter light has larger electric and magnetic field amplitudes, so each electron should gain more energy, and Kmax should rise with intensity. Experiment says it does not.
Also on the wave picture, a strong enough beam of any frequency, shining long enough, should give electrons enough energy to escape. There should be no threshold frequency. Experiment says there is one.
The wave spreads its energy over the whole wavefront, shared among a very large number of electrons, so each one gains energy only slowly. Calculations suggest that one electron could need hours or more to collect the work function.
That clashes with observation (iv): emission is instantaneous. The wave picture fails on observations (i), (ii), (iii) and (iv) alike: it cannot explain the basic features of photoelectric emission.