APPENDIX

APPENDIX . 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 spite of this, for over one hundred years, Newton’s particle model was firmly believed and preferred over the wave model. This was partly because of its simplicity and partly because of Newton’s influence on contemporary physics.

Then in , Young performed his double-slit experiment and observed interference fringes. This phenomenon could be explained only by wave theory. It was realized that diffraction was also another phenomenon which could be explained only by wave theory. In fact, it was a natural consequence of Huygens idea of secondary wavelets emanating from every point in the path of light.

These experiments could not be explained by assuming that light consists of particles. Another phenomenon of polarisation was discovered around , and this too could be naturally explained by the wave theory. Thus wave theory of Huygens came to the forefront and Newton’s particle theory went into the background. This situation again continued for almost a century.

Better experiments were performed in the nineteenth century to determine the speed of light. With more accurate experiments, a value of × m/s for speed of light in vacuum was arrived at. Around , Maxwell proposed his equations of electromagnetism and it was realized that all electromagnetic phenomena known at that time could be explained by Maxwell’s four equations. Soon Maxwell showed that electric and magnetic fields could propagate through empty space (vacuum) in the form of electromagnetic waves.

He calculated the speed of these waves and arrived at a theoretical value of . × m/s. The close agreement of this value with the experimental value suggested that light consists of electromagnetic waves. In Hertz demonstrated the generation and detection of such waves.

This established the wave theory of light on a firm footing. We might say that while eighteenth century belonged to the particle model, the nineteenth century belonged to the wave model of light. Vast amounts of experiments were done during the period - on heat and related phenomena, an altogether different area of physics. Theories and models like kinetic theory and thermodynamics were developed which quite successfully explained the various phenomena, except one.