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 made us 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 essential to summarize some observation about light that were known in the seventeenth century.
The properties of light known at the time include (a) rectilinear propagation of light, (b) reflection 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 1621. Several scientists right from the days of Galiteo had tried to measure the speed of light. But they had not been 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 Hungens, around 1650-60, proposed that light consists of wave. Descartes's proposal was merely a philosophical model, devoid of any experiment or scientific argument. Newton soon after around 1660-70, extended Descartes' particles 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 overs the next few centuries is interesting. Bartholinus, in 1669, discovered double refraction of light in some crystals, and Huygens, in 1678, was quick to explain it on the basis of his wave theory of light. In spits of this for over one hundred years, Newton's particles model was firmly believed and preferred over the wave model. This was partly because of its simplicity and partly because of Newton's influence of contemporary physics.
Better experiment were performed in the nineteenth century to determine the speed of light.
