The world of physics is a strange world indeed.
At end of nineteenth century it was found that a negatively charged metal plate would discharge when illuminated, but a positively charged one would not.
Then it was found by some researchers that NO metal plate would discharge.
Other researchers found that red light had no effect, but blue discharged plate.
The "photoelectric effect" was a mess.
Max Planck discovered that light consisted of particles. And each particle (or quantum) had an energy that could be defined as Joules.
The science was very simple. Take frequency of that light particle, multiply by Planck's constant, and you find how many Joules of energy are in that particle.
You could even pretend that this light particle collided with an electric particle - electron. If so, it would transfer its energy to that electron in just same way as chemistry of a battery energises electrons.
So we could define near infra-red as 1.7 electron volts, and at other end of spectrum ultra-violet begins at 3.2.
For this he got Nobel prize.
With electron-voltages of light being now defined, Einstein decided to tackle problem of metal plate. Why was it that only NEGATIVE electricity would be discharged? Because there is only one kind of "charge-carrier". Or so it seemed at time.
Why did some researchers find that NO metal plate would discharge? Because voltages are LOW. Even slightest film of grease or dirt could INSULATE against less than 3.2 volts.
And why did blue light work when red did not? The answer lay in Planck's electron voltages.
Einstein discovered that effect varied with TYPE of metal used for that plate. Each element, sodium, potassium, iron, copper, noble metals such as platinum &c., had a unique voltage.
That voltage represented energy that had to be present in light in order to free electrons.
That voltage became known as WORK FUNCTION.
Knowing this, scientists began to research forces that hold atoms together. In technical literature, Einstein's name was cited so often that Nobel Prize Committee could not ignore it.
So Einstein won Nobel Prize.
It is true that further researches modified simple model that scientists had used. For example, particle model of Planck suggested that an ultra-violet "PHOTON" could travel for thousands of millions of years through space, retaining its 3.2 eV energy - and NEVER become two photons at 1.6 eV each.
This is LAW OF CONSERVATION OF MATTER.
But after that huge journey, it needs only for that photon to hit a suitable crystal and it does indeed divide. You get TWO photons for price of ONE. This is only possible if photon consists of NOTHING.
So wave model of light began to supervene. Waves consist of nothing but a pattern. And questions were asked as to whether there is a SUBSTANCE in which waves are formed. Does space consist of aether?