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.
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.
