H Diffraction and Interference
  That light carries energy is obvious to anyone who has focused the Sun's rays with a magnifying glass on a piece of paper and burned a hole in it. But how does light travel and in what form is this energy carried?
Does light (em radiation) travel as Waves or as Particles?

During the 18th century most opinion sided with Newton. He had been right on so many things that it was generally assumed that he must be right about light being corpuscular.
Light is shown to demonstrate Refraction and Diffraction and Interference

Interference is a Wave Phenomenon
      Interference of two waves is a very simple concept. And, an even easier one to calculate. Simply add each point in the same place on each wave to get the resultant wave.

When two waves are both peaking at the same time in the same place they will Constructively interfere.

  If one wave is peaking in one direction and the other wave is peaking in the opposite direction they will
Destructively interfere

This concept of interference is incredibly important and philosophically rich.
    Particles and waves have different characteristics. Particles have 'zero' size. While waves have infinite extent. Particles cannot be in the same place at once. Two or more waves can be at the same location! This will lead to fantastic results.
    Animation courtesy of Dr. Dan Russell, Kettering University.
    Some very cool demos

Paul Falstad's Place
Young's Double Slit Experiment
      An explanation

The middle of the pattern is always light, and the bright stripes on each side are at such distances that the light coming to them from one of the apertures must have passed through a longer space than that which comes from the other by an interval which is equal to the breadth of one, two, three, or more of the supposed wavelengths, while the intervening dark spaces correspond to a difference of half a supposed wavelength, of one and a half, of two and a half, or more.
Fresnel won the prestigious Grand Prix Award for convincing evidence that light travels in wave and not particles
So much Interference, so little time!
Keck Interferometer 2, 3
Linear Superposition
Interference (Double-Slit)
Young's Double-Slit
Diffraction Grating
Thin Film
Compact Discs
Holographic Storage
Finally, it's Here!

Line Spectra of some Elements
The Big Bang
The centuries long investigation into light and it's fantastic properties has led us to The Big Bang. From discovering how light travels in the 'emptiness' of space, to the incredible speed of light, to the doppler effect and to the line spectra streaming away from dust clouds, stars and galaxies we have been able to coerce a mostly incomplete story of the beginning of the universe.
Interference in Thin Films

Holography and the Universe
Holography is a method for recording information onto film. This information is produced by interference of light waves reflected off the object with the reference light wave. Every part of the object is recorded in every location on the film. So, every part of the film has information about evdery part of the object.

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