I am myopic ~ -2.75 sph +1cyl. When I went snorkeling they tell you not to wear glasses behind the goggles. Surprisingly, underwater, things remain in focus with goggles on even without prescription lenses, while things outside the water at an equal distance would be blurry. Why is this the case?

Thank you.

  • 1
    $\begingroup$ I can't see squat when snorkeling without a prescription mask. $\endgroup$ – DanielSank Apr 9 '15 at 2:48
  • $\begingroup$ Some evidences suggest our eyes is designed to work underwater, however due to a weird but fun evolutionary path we emerged from water swimmer to land dweller. Anyway this is a fun fact lol drinks up. $\endgroup$ – user6760 Apr 9 '15 at 4:44
  • $\begingroup$ I agree with @DanielSank - my eyes are about -7, and I really need corrective lenses for my SCUBA mask or swim goggles to see clearly. $\endgroup$ – Jon Custer Apr 9 '15 at 13:26
  • $\begingroup$ Rich, I can assure you that when I was at approximately a -10 diopter reading in glasses, and -8.5 diopters in contact lenses, I definitely didn't see things clearly underwater. I had lasik some time in the past, so that problem is fixed. $\endgroup$ – David White Aug 29 '18 at 19:56

the assertion that corrective lenses are not needed underwater is incorrect. diving masks with corrective lenses built-in have been in use for decades. the reason you yourself experience this effect is probably contained in Chris' and Martin's comments above.


Wearing the mask underwater doesn't do anything to enhance your vision, but it does make objects in the water appear larger/nearer. This is due to the refraction of light at the air/mask interface (more info in my answer to this question). The objects are magnified by a factor approximately equal to the ratio of the indices of refraction $\frac{n_{water}}{n_{air}}\simeq1.33$, so you are effectively able to see $1.33$ times as far.

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  • $\begingroup$ Also under water you can't see as far, visibility is at best 10m, and you aren't reading road signs or anything else in detail - so "in focus" is a bit less strict $\endgroup$ – Martin Beckett Apr 9 '15 at 20:44

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