Three men having normal vision, hypermetropia and myopia repetitively, jumps into lake for finding a diamond ring. Who had a better chance of spotting the ring?
All the three men have their naked eyes directly exposed to water.
The myopic diver has the best vision:
On land, the normally-sighted swimmer's eyes take advantage of the difference in the index of refraction between air 1.00 and the eye's vitreous humour fluid: index 1.34 (and cornea index 1.41), to bend the light to a focus on the retina.
This bending is largely absent underwater since there is little difference at the interface: water index 1.33, leaving the normal-sighted person far-sighted (image formed behind the retina, and out of focus on the retina for objects at medium distances).
But the myopic person's eyes have too long a path to the retina, or their cornea is too curved, and instead focus the image in front of the retina, leaving it out of focus on the retina. This focus is bad for normal (in-air) seeing, yet helpful underwater, where the water and fluid interface cause little bending. So the myopic lens and length pick up the burden and serve to better focus the image further back, nearer the retina.
The far-sighted diver's seeing is worsened underwater.
Likewise, presbyopia, the inability to focus near due to the hardening and inflexibility of the cornea, afflicting many older people (degradation starting ~40 and stopping ~65), also diminishes underwater seeing. The fix: goggles or mask.
But for the diamond, I would also place my bets on the one who can swim better and hold their breath longer. :-)
Edited on March 5, 2019, 1:02 am