Background: 

Behavior of light 

There have been many theories over time to understand the mystical nature of light. Light is a collection of small packets of photons that show electromagnetic wave properties. This means that unlike mechanical waves (for instance sound) light can propogate through a vacuum without losing any energy. 

Until the late 17th century, light propogation was considered to be linear. Christiaan Huygens first proposed the wave nature of light propogation, and this understanding was further divided into longitudinal and transverse wave theory. Light transmits energy in quantum photons, which means that they posess momentum. In order to better understand this, it is worth reading up on Plank's Quantum Theory. 
 

 

 

 

 

 

 

 

 

 

 

fig. 1 Properties of light

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Different properties of light can be understood through two light propogation phenomena -- Linear and Wave. Reflection and refraction can be easily understood through the linear nature of light, while diffraction, interference and polarization can be understood through its wave properties. By the way, waves aren't even real -- waves are just a disturbance in medium, the particles in the medium move to and fro but never change their mean position (fig. 1). 
 

Light Sources

• Monochromatic

• Polychromatic

• Coherent

  • Single: Same phase

  • Double: Constant phase difference

Wave Fronts

• Imaginary Plane drawn for coherent source only

• Locus of points in same phase

• Two wavefronts of same source never intersect

• Perpendicular to wave propogation

• Parallel to each other

 

 

 

 

 

 

 

fig. 2 Wave superposition

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Thin Film Interference 

Thin film interference occurs when light reflects from and refracts through a very thin layer of transparent or translucent material which has a diferent refractive index than the surrounding media. The interference is caused due to multiple bounces of light within the thin film media. These refracted light rays interfere with the first reflected light. Thus based on the wavelengths of interfering light we see a colorful pattern on the surface of the material. This is the color pattern we are trying to reproduce. 

The interference pattern is a result of phase shift, due to Optical Path Difference, which is calculated as follows: 
Optical Path Difference = (AB + BC) - AD 

In the case that the film is denser than the surrounding media (n1 > n0) R0will have a phase shift of π. 

This causes light and dark intensity bands, as seen in Young's Double Slit Experiment. In the case of thin film interference, the interference will be different at different wavelengths of the light. If the incident light is monochrome we will only see dark and light bands of that particular color and not a colorful pattern.