# The History of wave-particle flip flop

What is light? A particle or a wave? This question has hunted physicist from centuries. The story of dual nature of light is quite interesting. It took more than three centuries to answer the most basic question. While developing a model in science it is essential that it is able to explain all the known phenomenon at that time. In 17th century the rectilinear propagation of light, reflection of light, refraction of light, the dispersion into various colors and speed of light were the chief.

## 17th Century Debates: Particle vs. Wave Models of Light

Two different model of light the particle model and the Wave model were proposed in 17th century. Descartes in early decades proposed the particle model while Huygens around 1650-60 proposed the wave model of light.

The particle model of Descartes was merely a philosophical model devoid of any experiments and scientific theory. But the model was supported by Newton who put forward the corpuscular model of light in which the light consists of small corpuscular or particles. The particle model was able to explain the basic reflection, refraction and rectilinear motion of light at time and hence became widely accepted at that time. Also because everyone had high esteem and respect for Newton no one believed in the wave theory of light.

Huygens proposed the wave theory of light. He said that light is a wave consisting of wave fronts moving in forward direction. Due to the prevailing model of light and its wide acceptance the wave model did not became famous. However in coming years it was only the wave model that could have explained the upcoming physics of the time.

## Young`s double slit and Wave theory

In 1801, Young performed his double slit experiment and changed the course of physics for once and for all. Double slit experiment became the basics for the quantum theory and it is more subtle to understand it through the classical physics. Still the only the wave theory could explain the phenomenon and the particle theory of Newton went into the background.

Adding to that Maxell in 1860, proved that light is an electromagnetic wave with speed of these waves to be 2.998 x 10^8 m/s. Because light is an electromagnetic wave it does not need a medium to travel and hence can travel through empty space. During that time phenomenon of diffraction and polarization also came to forefront in physics which could only be explained by the wave theory of light. We might say that while the eighteenth century belonged to the particle model, the nineteenth century belonged to the wave model of light.

## Heat to Quanta: Physics’ Unanswered Questions

Vast amount of experiments done in 1850-1900 mainly focused in the kinetic theory and heat related phenomenon in physics. Everything was well and good. It seems like physics was able to explain almost everything at that time but one phenomenon remain unsolved. The radiation spectrum of a black body. An ideal body which absorbs and emit all the radiation which falls on it. It is an ideal concept in physics, like concept of uniform mass. The spectrum showed that the energy of body tends to be infinite with increasing frequency. Which is opposite of what happens in reality.

In 1900, Max Planck hit upon a novel idea. He proposed that that energy of radiation comes in discrete packets called ‘quanta’. Hola!!! Particle model`s reentry. Now according to this model the radiation is emitted as a particle, it travels as a wave and is again absorbed as a particle!

But soon in 1905, Einstein explained the photoelectric effect using the particle model of light. In 1907, Debye explained the low temperature specific heats of solids by using the particle picture for lattice vibrations in a crystalline solid. Both the phenomenon could only be explained by the particle model of light. In 1923, Compton`s x-ray scattering experiments from atoms also went in the favor of the particle picture. This increased the dilemma further.

Somebody in those days aptly remarked that light behaved as a particle on Mondays, Wednesdays and Fridays, and as a wave on Tuesdays, Thursdays and Saturdays, and we don’t talk of light on Sunday!

In 1924, De-Broglie proposed his theory of wave particle duality in which he said not only photons of light but electron and other atoms possess a dual character. He gave a formula relation the momentum, mass and velocity which are properties of particle to frequency and wavelength (wave characteristics). The theory was based largely on the symmetry of nature. Further in 1927 Thomson, Davidson and Germer, in separate experiments showed that the electrons did behaved like waves with wavelength as predicted by the De-Broglie hypothesis. Here was the principle which physicist thought, which explained all the phenomenon mentioned above not only for light but also for the so-called particles.

In conclusion, the journey to understand the nature of light evolved from conflicting models to the realization of wave-particle duality. From Descartes’ particles to Huygens’ waves, Young’s experiment, Maxwell’s electromagnetic waves, Planck’s quanta, and De Broglie’s dual nature, the intertwined dance of light and matter led to a unified understanding of fundamental physics.