I can’t see clearly when I open my naked eyes underwater. Is this always the case? Or is it just me?

Theoretically, people with normal vision will never be able to see close objects clearly with the naked eye underwater.

"Seeing clearly" is a vague concept. "Seeing clearly" is defined here as: seeing things to the same extent as underwater photography or wearing diving goggles.

To see objects clearly, light must be clearly imaged on the retina. That is to say, the diopter is fixed, and there are two factors that change the diopter. One is the shape of the lens, and the other is the refractive index ratio between the human eye lens and the external medium. Because the lens medium is not uniformly distributed, the exact calculation of this problem may be complicated, but we can discuss it more simply.

1. Assume that the human eye lens is only composed of a uniform medium, and its refractive index is approximately 1.42 (refer to the optical constants of the lens - Ophthalmology News - Eye Care Center - World Eyewear Network)

2. The refractive index of water is about 1.3, which will increase with the increase of salinity in seawater.

Because the refractive index of the human eye and water are close, if the shape of the lens is not changed, the light will be imaged behind the retina, making it impossible to see clearly. There is only one way for people to see things close up underwater with the naked eye: high myopia. (Further, you can actually calculate how much myopia is enough.)

Author: Fu Shishui

Link: /question/20196979/answer/14985046

Source ：Zhihu

The copyright belongs to the author. For commercial reprinting, please contact the author for authorization. For non-commercial reprinting, please indicate the source.