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The article I was reading states that one factor affecting the occurrence of thin-film interference is the light source. The quality of the light reflecting off a film can impact the amount of iridescence observed. The “whiter” the light is, meaning the closer it comes to emitting all colours in the visible light spectrum, the less iridescence will be observed.

Can anyone help to explain why the phenomenon of thin-film interference occurs in a more detectable manner when the light source is emitting only certain wavelengths of light (e.g. fluorescent lights) compared to a higher quality light source emitting all wavelengths in the light spectrum (e.g. LED light, natural sunlight)?

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  • $\begingroup$ I think coherence (coherence length) of the light source will be the most important factor to observe interference effects. Are you familiar with this? $\endgroup$
    – Cryo
    Commented May 18, 2020 at 8:51

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Monochromatic light is light of a single wavelength, or at least very narrow bandwidth. This makes for very sharply defined interference bands. This is pretty much essential for a good two slit result.

As it is usual to use lasers for such experiments, the light will also be coherent. That is, all the photons in the light are in phase across the beam and the phase changes smoothly along the beam.

This means that the interference pattern is stable, not moving around as different phase photons interfere. Again this is pretty much essential for a good two slit result.

A laser pointer is a good source of coherent monochromatic light. If you hold three pencil lead in front of the beam you should be able to show a good set of interference bands.

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The phase difference depends on the wavelength of light. So the different coloured interference patterns don’t overlap perfectly. enter image description here

This makes it harder to resolve between intensity peaks reliably.

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Actually, the most important thing about the light source in thin-film interference is its spatial coherence: the range of angles at which it illuminates a point on the thin film. If that range is small, (and if the spectral bandwith of the source covers the visible spectrum), then the colors due to thin-film interference are easy to see. If that angular range is not much narrower than the angular spacing between interference colors, then the colors will mix.

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