Interference of light waves become observable when we have narrow slit separation. Surprisingly, nature provides us with many natural double slits and gratings.

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Those shimmering colours you see reflected off a thin layer of oil is a double-slit interference pattern. The light reflected off the top surface and the light reflected off the bottom layer (total internal reflection) interfere with each other.

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Some shells are iridescent. Thanks to regularly spaced surface grooves. The light reflected off the grooves act as regularly spaced light sources.

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Butterflies too. Thanks to the regularly spaced scale structure on their wings.

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Compact discs, are man made objects of course. It has regularly spaced and finely packed grooves, which produces spectacular interference patterns.

• 10 Superposition