Colourful thin film interference on Anodized BismuthBela VarsaniAward: Top 100
School: queensland academy for science mathematics and technology Often, we see colours as being primarily aesthetic. However, in the world of physics, colours tell us a lot about a phenomenon. This photo was taken after completing an experiment measuring the breakdown voltage of bismuth anodization at a constant current with varying concentrations of sodium hydroxide. The breakdown voltage is the voltage at which the critical thickness of the oxide film is obtained, and electrons are dislodged in the film resulting in the film's destruction. As the oxide film has a different refractive index to air, thin film interference occurs. The light waves encounter the oxide layer where part of the wave is reflected, and part is refracted. The refracted ray travels to the bismuth where it changes phase and is reflected. The wave constructively interferes and produces the visible colours we see. When anodized at a constant voltage the colour is constant. However, when the film is destructed, it has an uneven oxide thickness. This forms the various colours seen in this photo due to the dependence of wavelength on thickness. The main colours change with the varying voltages the film's broke at and their corresponding thicknesses, both of which relate back to the changing electrolyte concentrations. 
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