Victorian physics treated energy, heat, light, space, gravity, and other phenomena as continuous. If one had a certain amount of space or light, one could as easily have half that amount. It was infinitiely divisible into finer units. Planck made the astonishing discovery that a certain calculation could only be done if radiant heat was not continuous but came in little bundles. He called them "quanta." Einstein--exactly a century ago--said that light, too, came in little bundles. We call them photons today.

There was a bit of a problem. Light is also seen to behave like a wave in some experiments. For instance, white light splits into colors in a prism. The final situation seems to be that each photon is a little "wavelet" with intensity varying a its little space. That intensity can be described with the same sort of equation that can describe water ripples where a rock has fallen through. Ripples further from the center are smaller. In the same way, the ripples of a wavelet fall off in intensity at distances from the center of the photon. They fall off much more rapidly so that the photon appears to be pretty much at one place.

About the same time, other particles were discovered; notably electrons, neutrons and protons. Everything tangible that we know of is made of these as they are grouped into atoms. An atom is a nucleus of neutrons and protons surrounded by a cloud of electrons. The electrons are gathered to the nucleus because they have negative electric charge and the protons in the nucleus have positive charge. There are always more neutrons than protons and these neutrons play a role in preventing the protons from themselves flying apart and scattering the atom.

Further experiments show that all particles are capable of wave-like behavior. Schroedinger developed his Wave Function to describe the intensity of any given particle at each point surrounding its putative center. Thus all particles--all of physical existence--came to be understood as instances of quanta.

The "Wave-particle Duality" notion expresses the situation by stating that every particle, not just photons, can be treated as either a wave or a particle. In some experiments the objects behaves like a particle. For instance, it bounces. In other experiments it behaves like a wave. Usually this is observed as the particle "interfering" with itself.

We can see interference of photons in most roadway puddles, as beautifully explained by Dinesh O. Shah.

Interference occurs when two light waves arrive at the same point. If they are both at their highest, the values add and the light at that point is brighter. this is in the top half of the diagram. But if one is high and the other low, they counteract each other and there is no light. If the light is a single color, the result will be dark or that color, as shown in the bottom half of the diagram

In the puddle the incoming sunlight is all colors and the oil film varies in thickness. This happens for all colors, each with its own wavelength. The wave bouncing off the top of the oil film has a different travel time than that from the back, so the peaks of the waves have diferen relations at each point. Some colors cancle out and others are enhanced.