CSEC Physics: Light and Wave-Particle Duality

For as long as humans have been curious about the nature of their environment, there have been theories about the nature and origin of light.

The first theories surrounding light were established in Ancient Greece, where research on optics was carried out between the 5th and 3rd centuries BC. Greek mathematician Euclid summarized the fundamental knowledge on optics at the time, including reflection and diffusion and their relation to vision. The theories around this time considered colours to be a mixture of light and dark. Plato postulated that light consisted of rays emitted from the eyes, known as the extramission theory of vision.

In the 17th century, two major theories on the nature of light were presented, one by Isaac Newton, and the other by Christiaan Huygens.

Newton is a defining figure in scientific discovery. His publication Principia laid down the foundation of classical mechanics, his co-discovery of calculus (with Wilhelm Leibniz) provided tools essential for subsequent discovery in physics, and his law of universal gravitation provided great insight into the nature of our universe.

Curiously enough, the most important experimental contributions to physics made by Newton are all in the field of optics. He was the first to show that colour is the property of light and not of the medium. For example, when sun light passes through a prism, it is dispersed in a rainbow of colours. The red colour bends the least and the violet colour bends the most. Although this property was known since antiquity, it was Newton who proved that colour was not a characteristic of the material. When one colour was passed through a prism, no dispersion of colour occurred. However, when the dispersed white light from one prism was passed through another, white light was recovered. Thus, white light was shown to contain all colours.

Newton was also concerned with the nature of light. His theory, known as the corpuscular theory of light, proposed that light consists of small particles called corpuscles. These particles are far smaller than those in ordinary matter, which consist of larger corpuscles (according to the theory). Newton's corpuscular (particle) theory of light allowed him to account for refraction and reflection using his laws of motion. However, there was mounting evidence for the conflicting theory that light is a wave. For example, an observation made by Francesco Grimaldi of the phenomenon known as the diffraction of light proved otherwise. Through an experiment, he showed that light, when passed through a singular hole, did not follow a linear path as would be expected if it were a particle. Instead, it took on the shape of a cone, which is the diffractive behaviour of a wave.

Christiaan Huygens postulated that light was a wave rather than a particle. He believed that light consisted of waves moving up and down propagating through media or a vacuum. He suggested that light waves peaks formed surfaces that would propagate and spread out as they passed through a medium. This explained why light would spread out (or diffract) after being passed through a small hole.

Despite Newton's incredibly high status as a scientific mind, Thomas Young still challenged his theory of corpuscular light, and conclusively demonstrated the wave nature of light through his double-slit experiment. Because he believed that light was composed of waves, Young reasoned that some type of interaction (interference) would occur when two light waves met.

(Go to this site to see a great interactive tutorial of the experiment and its results)

Using sunlight diffracted through a small slit as a source of coherent illumination, he projected the light rays coming from the slit onto another screen containing two slits placed side by side. Light passing through the slits was then allowed to fall onto a screen. Young observed that when the slits were large, spaced far apart and close to the screen, then two overlapping patches of light formed on the screen. However, when he reduced the size of the slits and brought them closer together, the light passing through the slits and onto the screen produced distinct bands of colour separated by dark regions in a serial order. Young coined the term interference fringes to describe the bands and realized that these coloured bands could only be produced if light were acting like a wave.

It seems contrary, right, for the addition of more light (from a second aperture) to result in less illumination. However, the only explanation would be for there to be some form of wave interference (like what happens to two ripples on a lake).

Please look at our previous post here if you are unfamiliar with wave interference.

Einstein in the 20th century, proposed that light is composed of photons, very small packets of energy. The reason that photons are able to travel at light speeds is due to the fact that they have no mass and therefore, Einstein's infamous equation E=MC^2 cannot be used. Another formula devised by Planck, is used to describe the relation between photon energy and frequency - E= hf or E = hc/1

Planck's Constant (h) - 6.63x10-34 Joule-Second and speed of light (c)=299792458 m/s

The photon exhibits both the properties of a wave and a particle, thus, light is said to have a wave-particle duality.

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