Interferometry using natural light
It is commonly said that lasers are sources of "coherent" light and that thermal sources (the sun, candle flames, incandescent light bulbs, etc.) are sources of "incoherent" light. This notion is bolstered by the relative ease with which we can produce high contrast interference fringes using laser light (indeed, diffraction from dust particles is often apparent whether we want it our not). More fundamentally, as we observe the world illuminated by the sun and other broadband sources, we do not generally observe interference effects.
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