In the late 1600s, important questions were raised about the properties of light. Although many experiments concluded and supported the wave theory of light, there was also several experiments that contradicted wave theory, but showed light behaving as a particle.

Sir Isaac Newton, held the theory that light was made up of tiny particles. In 1678, Dutch physicist, Christiaan Huygens, believed that light was made up of waves vibrating up and down perpendicular to the direction of the light travels, and therefore formulated a way of visualising wave propagation. This became known as 'Huygens' Principle'.

Huygens theory was the successful theory of light wave motion in three dimensions. Huygen, suggested that light wave peaks form surfaces like the layers of an onion. In a vacuum, or other uniform mediums, the light waves are spherical, and these wave surfaces advance or spread out as they travel at the speed of light. This theory explains why light shining through a pin hole or slit will spread out rather than going in a straight line (see diffraction). Newton's theory came first, but the theory of Huygens, better described early experiments. Huygens' principle lets you predict where a given wavefront will be in the future, if you have the knowledge of where the given wavefront is in the present.

At the time, some of the experiments conducted on light theory, both the wave theory and particle theory, had some unexplained phenomenon, Newton could not explain the phenomenon of light diffraction or interference, this forced Newton's particle theory in favour of the wave theory. This difficulty was due to the unexplained phenomenon of light polarisation, scientists were familiar with the fact that wave motion was parallel to the direction of wave travel, NOT perpendicular to the to the direction of wave travel, as light does.

In 1803, Thomas Young studied the interference of light waves by shining light through a screen with two slits equally separated, the light emerging from the two slits, spread out according to Huygen's principle. Eventually the two wave fronts will overlap with each other, if a screen was placed at the point of the overlapping waves, you would see the production of light and dark areas (see interference).

Later in 1815 & 1818, Augustin-Jean Fresnel elaborated further on theories advanced by Huygens and further supported Young's experiments with mathematical calculations.

In 1862, James Clerk Maxwell began explaining the propagation of electromagnetic waves (and consequently how light propagates), and in 1865 published his set of (originally) eight equations explaining his theories. These equations became known as Maxwell's equations.

In 1887, Heinrich Hertz conducted experiments that supported Maxwell's work.

From these early theories and experiments, wave theory of light became widely accepted.

In 1900 Max Planck proposed the existence of a light quantum, a finite packet of energy which depends on the frequency and velocity of the radiation through his studies of black-body radiation and development of his (Planck) constant.

In 1905 Albert Einstein had proposed a solution to the problem of observations made on the behaviour of light having characteristics of both wave and particle theory. From work of Plank on emission of light from hot bodies, Einstein suggested that light is composed of tiny particles called photons, and each photon has energy.

Light theory branches in to the physics of quantum mechanics, which was conceptualised in the twentieth century. Quantum mechanics deals with behaviour of nature on the atomic scale or smaller.

As a result of quantum mechanics, this gave the proof of the dual nature of light and therefore not a contradiction.

In the next section, we will cover wave theory in more detail.

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