# Optics Lessons: Part 2 - The Dual Nature of Light

The nature of light has been a controversy debated throughout the course of history. Two models evolved: the wave model and the particle model. Each model has taken an important dominating place, but neither has been disproved completely. Therefore, the scientific community has combined the two theories to form the wave particle duality, where characteristics from both models are attributed to light (the “wavicle”). These two seemingly contradicting models have been reconciled by a new theory of physics, called quantum electrodynamics, which is a quantum theory of the electromagnetic interactions between photons, electrons and muons (an elementary particle having a mass around 200 times greater than an electron). This model is also known as quantum theory of light.

## The Wave Theory of Light:

The theory that light is transmitted via wave form was proposed by Christian Huygens in 1679. According to his theory:

• Light must travel through a medium.
• This medium is called Ether, which is an omnipresent, boundlessly resilient, massless medium unable to be sensed that was formerly theorized as the medium that carried light.
• Light is a form of energy - Light travels in waveform in short wavelengths.

## The Particle Theory of Light:

The theory that light is transmitted via corpuscular form was supported and advocated by Sir Isaac Newton. According to this theory: light is composed of particles (“corpuscular”) that travel in a straight line at an extreme speed, they allow the eye experience vision when they are absorbed, and various colored particles all have differing sizes.

## The Quantum Theory of Light:

The theory that combines both the wave and particle models was suggested by Max Planck According to this theory: light is absorbed or released in bundles of energy called photons.

## Huygen's Principle:

Christian Huygens, a Dutch scientist, supported the concept that light is a wave motion that is transmitted through ether. Applying this theory, Huygens derived the laws of reflection and refraction.

## Young's Double-Slit Experiment:

Thomas Young (1773-1829), an English scientist, also credited for helping translate the Rosetta Stone, conducted an experiment in 1801, known as Young's double-slit experiment, that allowed him to show the wave nature of light and measure its wavelengths. The setup of this experiment is quite elegant--Light from a single source shines through two narrow, closely-placed slits for them to serve as two coherent light sources. (A coherent light source is one that makes light waves have a consistent phase relationship with each other--crest overlaps crest, trough overlaps trough) A screen is put behind these slits.

Now, if light consisted of particles, then the screen would reveal two bright lines when the light illuminated through. Young, however, saw a center bright line and groups of symmetrical dark and bright edges and explained these groups to be marks of constructive and destructive interference.

So, since we've covered the basics, here is a diagram, which geometrically analyzes the experiment and explains some equations:

As the image indicates, the difference in the length of and is d sin().

Equations for Young's Double-Slit Experiment may be found here.

## Fresnel and Transverse Waves:

Augustin Fresnel, in 1821 put forward results from experiments involving double refraction and polarized light suggested that light is a transverse wave, which is a wave where the vibrating part moves perpendicularly to the coming wave. Equations that he formulated, the Fresnel equations provide the amplitude of reflected and transmitted polarized light traveling at a plane interface.

## Further Developments:

James Clerk Maxwell, a scientist studying electricity and magnetism, formulated the four Maxwell equations, which gave a predicted speed of an electromagnetic wave through ether, which actually ended up being the speed of light, c. So from this point on, light was considered to be a part of the electromagnetic spectrum.

And later still, Einstein's theory of relatively helped throw out the idea of ether, which solved some problems dealing with the transverse vibrations of waves in liquids and gases.