The quality or state of being diffractive; the degree to which something is capable of or tends to cause diffraction.
From 'diffractive' plus the noun suffix '-ness,' which creates abstract nouns indicating a quality or state. This suffix is one of the oldest in English.
The 'diffractiveness' of different materials tells engineers which ones to use for precise optical instruments—some materials bend light waves much more elegantly than others, which is why your expensive camera lens works so well!
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