Before the beginning of the twentieth century, scientists had a firm belief that the physical world is divided into two worlds: the world of particles, and the world of waves.
The consensus was that the material world is composed of little, round objects, atoms and molecules that interact with one another to produce the variety of materials, living and non-living, we see around us. They also had a very good theory of how light propagates, in the form of electromagnetic waves, in many ways analogous to the ripples on a pond, or the sound waves that carry information in the form of vibration in the air. In short, the end of theoretical physics and the solution of all the great puzzles seemed to be in sight.
However sometime at the beginning of the last century, the house of cards
so painstakingly constructed came tumbling down. It turned out that the behavior of light
could sometimes be explained in terms of particles called photons while the wave
explanation or model, remained the only valid one in other circumstances. A little later,
physicists realized that if waves that behave sometimes as particles were not enough to
worry about, particles sometimes could indeed behave as waves!