Concepts explained
Particle or Wave — The Answer Is Both
Newton said particle, Huygens said wave. The wave won, then was overturned a century later. Today the answer is both.
That is what waves do. Drop two stones into still water and a pattern appears where the ripples meet: crest on crest builds, crest on trough cancels.
That experiment closed the argument. Light was a . Newton was wrong.
A century later, a metal plate overturned it
Shine light on metal and current flows: light knocks electrons out. On the wave account the answer is obvious. Brighter light hits harder, so electrons should leave faster.
They do not. However bright the red light, nothing comes out. Very faint violet light, and out they come at once.
The 1905 explanation ran thus. Light does not arrive as one spread-out wave but in lumps. One electron meets one lump. What a single lump carries is set by colour. Red lumps, however many, are each too weak to prise an electron loose.
The man who broke the wave theory and brought the grain back would later take a Nobel for it — for this, not for relativity.
A little further in
So the answer is both
Light spreads like a wave while it travels and lands like a grain when it strikes. Both experiments are honest and both are right.
It overlaps through two slits and lays down stripes, bends around corners, refracts in water. This is light while it travels.
It knocks electrons out one at a time; a sensor counts it grain by grain. This is light at the moment of exchange.
It sounds contradictory because everything familiar is one or the other. A baseball is a grain, a swell is a wave. Light is neither. There is nothing nearby to compare it to.
The strangest version of the experiment
Run the two-slit experiment firing grains of light one at a time. One, wait, another. A single grain has nothing to overlap with, so no stripes should form.
Let it accumulate and the stripes appear. As though one grain went through both slits at once.
Worse: install a device to watch which slit it took, and the stripes disappear. Stop watching and they return.
From here on it is quantum mechanics, and the library series 'Quantum Mechanics, In Order' begins at precisely this spot.
The question that remainsIf light is grain and wave at once, is the distinction between 'grain' and 'wave' a fact about the world or a fact about our heads?