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Refractive indices

To see underwater just like on the shore with your regular glasses you'll need corrective lenses for your diving mask. However, simply making these lenses according to your regular prescription glasses is not always sufficient. Underwater some different refractive rules apply. Also the conditions are different underwater then they are on land. As is (in most cases) the field of use.

All these factors can lead to your regular prescription needing some adjustment to get the best results.

On this page you will find some information on these matters. They serve an informative purpose only so no liability is being taken for the (correct) implementation of this information.

Air has a refractive index of 1
Glass has a refractive index of 1.5 to 1.8
Water has a refractive index of 1.33

In other words: rays of light will refract differently (less) at the transition plane water | glass (1,33:1,5) then at the plane air | glass (1:1,5).
This would mean the diving mask lenses have to compensate for this 'loss of refraction' resulting in higher refracting powers. The need to do so is more of an issue with close / reading distances then at mid or far range distances.