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Why is optical contact called optical contact? whats the meaning of the 'optical', when the concept of optical contact bonding has nothing to do with optics.

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This phenomenon is practically exclusively applied in optics. It is used for binding optical elements together. It requires extreme flatness, or more generally conformity, and cleanness of the surfaces to be bound. Such conditions are in practice only met in optics.

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If you put two faces of glass in contact with each other, you would expect that light would not reflect from the interface provided that the refractive indexes are the same. But usually light does reflect because any roughness, dirt, or non-parallelism will cause a small gap between the glass faces, which the light “sees”.

The idea of optical contact is that the interface is smooth, clean, and close enough that light sees it as an ideal interface. If there are two materials of the same refractive index in optical contact, light doesn’t recognize that there is an interface at all.

Actually, the easiest way to get optical contact between two surfaces is with index-matched oil. So the surfaces don’t have to be perfect; a fluid can fill the gaps. This is what is used for microscopy or total-internal-reflection spectroscopy, for example.

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