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enter image description here Why the silicon nitride colour varies with the thickness?

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The colours are formed by thin film interference, much like a soap bubble or an oil slick. Light is partially reflected off of the front surface of the coating and also off of the back surface. The two reflected light waves experience a difference in path length of twice the thickness of the film. This leads to light interference which can be constructive, or destructive depending on the thickness of the film and the wavelength of the light. When illuminated with a mix of wavelengths, such as in white light, you will get constructive interference for some colours and destructive for others. The combination of which wavelengths are most strongly reflected varies with the thickness of the coating.

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