Optical physics and the cyclops

[snowgoat]

When diving with my Sub Date I have never once looked at the date, but today in the swimming pool was surprised to see the the date magnification is void as soon as the watch is immersed. Even if my head was also submerged, rather than viewing through the surface.
Seems a silly question perhaps but my curiosity is piqued and my light physics days are long behind me. Can anyone enlighten me as to why. I’m guessing that it’s in part due to diffraction gradient between the lens and air compared to lens and water.... but I may have oversimplified. Had the watch 5 years and never noticed it before.

[Kowlick]

Originally Posted by snowgoat

I’m guessing that it’s in part due to diffraction gradient between the lens and air compared to lens and water....

I think you've got the right idea. I believe the magnification is due to the ratio of refractive indices of sapphire to air vs. sapphire to water. Sapphire has an index of refraction (IOR) of 1.77, water has an IOR of 1.33, while air is 1.0. Because the ratio of sapphire to air is higher than sapphire to water, light refracts (bends) more at the interface between the sapphire and air, resulting in greater magnification.

I'd still expect some level of magnification in water though, unless the water and sapphire had the same IOR. In the case of two materials with the same IOR, there is no refraction and therefore no magnification.

[77T]

Divers can usually remember the date whist underwater

Don’t know how you were viewing the watch underwater - but with goggles or a dive mask the cyclops works as expected.


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[snowgoat]

Thought it must be something to do with diffraction... thanks

And agreed who needs the date when diving.

It was viewed through standard swimming goggles and I believe that since the surface for diffraction is the cyclops water interface whether viewed under water with or without goggles it would make no difference to the magnification which I what seemed so interesting when I first noticed it.