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introduction

In the past 400 years alone, from Thomas Young to Albert Einstein, there have been a range of theories regarding the nature of light. While some experiments prove that light acts like a wave, and others prove that light, we’ll be taking a closer look at thin film interference and single slit diffraction to prove that light behaves as a wave.

thin film interference

<aside> 💧 Occurs when light waves reflecting off the top and bottom surfaces of a thin film interfere with one another

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• In thin films such as oil or soap bubbles, there are colourful rainbow-like patterns due to changes in wavelength

• Light waves become inverted when they reflect from the boundary of a medium that has a higher index of refraction than the original medium

• Light waves that reflect from two surfaces of a thin film produce interference fringes that depend on:

  • Different path lengths travelled by the two waves
  • Wavelength of the light
  • Phase changes that occur from reflection
  • Indices of refraction for the materials involved

• If only one of the waves undergoes a phase change by $\pi$:

  $2t = \frac{(m+\frac{1}{2}) \lambda}{n_{film}}$ for $m = 0, 1, 2, 3$ in constructive interference

  $2t = \frac{n \lambda}{n_{film}}$ for $n = 1, 2, 3$ in destructive interference

  If both waves have a phase shift, the equations for constructive and destructive interference are swapped

what would have happened with a particle?

If particles were to interfere with one another, they would bounce off each other in random directions

• Particles are unable to form constructive or destructive interference patterns because they cannot occupy the same space at the same time
• They cannot interfere in the same manner that light does

single slit diffraction

<aside> 〰️ Monochromatic light passing through a single slit produces a diffraction pattern that is a central bright region surrounded by light bands as a result of constructive interference, and dark bands from destructive interference

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• Similar to double slit diffraction, except that the light spreads out from a line that is perpendicular to the slit it passed through

  • Produces a diffraction pattern rather than an interference one