A PPENDIX . The history of wave-particle flip-flop What is light? This question has haunted mankind for a long time. But systematic experiments were done by scientists since the dawn of the scientific and industrial era, about four centuries ago.
Around the same time, theoretical models about what light is made of were developed. While building a model in any branch of science, it is essential to see that it is able to explain all the experimental observations existing at that time. It is therefore appropriate to summarize some observations about light that were known in the seventeenth century. The properties of light known at that time included (a) rectilinear propagation of light, (b) reflection from plane and curved surfaces, (c) refraction at the boundary of two media, (d) dispersion into various colours, (e) high speed.
Appropriate laws were formulated for the first four phenomena. For example, Snell formulated his laws of refraction in . Several scientists right from the days of Galileo had tried to measure the speed of light. But they had not been able to do so.
They had only concluded that it was higher than the limit of their measurement. Two models of light were also proposed in the seventeenth century. Descartes, in early decades of seventeenth century, proposed that light consists of particles, while Huygens, around - , proposed that light consists of waves. Descartes proposal was merely a philosophical model, devoid of any experiments or scientific arguments.
Newton soon after, around - , extended Descartes particle model, known as corpuscular theory , built it up as a scientific theory, and explained various known properties with it. These models, light as waves and as particles, in a sense, are quite opposite of each other. But both models could explain all the known properties of light. There was nothing to choose between them.
The history of the development of these models over the next few centuries is interesting. Bartholinus, in , discovered double refraction of light in some crystals, and Huygens, in , was quick to explain it on the basis of his wave theory of light. In