Physics trivia

Why can’t we see well underwater?

Our eyes cannot focus on objects underwater due to the proximity of the refractive indices of water and the interior of the eyes.

If you’ve ever tried to look for something underwater, you’ve probably already encountered the great difficulty we have to see in this situation. Some structures present in the human eye have characteristics that make it difficult to see in certain optical media .

refraction of light

In a vacuum, light travels at its limiting speed (approximately 3.10 8 m/s ). When entering the water, however, its speed decreases by about 25% . This happens because of a phenomenon called refraction : the more refractive an optical medium is, the slower the speed of light inside it. The decrease in the speed of light is measured by the ratio (division) of the magnitude of the speed of light in vacuum and the speed of light in a given medium . . This ratio is called the refractive index and indicates how many times faster is the propagation of light in a vacuum in relation to the observed medium. Water, for example, has a refractive index of 1.33; in this case, the speed of light in a vacuum is 1.33 times greater in a vacuum than in water.

In addition to the change in speed, when light changes optical media with different refractive indices, it can undergo a change in its propagation direction , giving rise to the illusion of depth that we have when looking at the bottom of pools, which appear more . than they really are. When light leaves atmospheric air and falls on water, which has a higher refractive index, for example, its direction is slightly altered and approaches a vertical line called the normal line.

This happens with the light that falls on our eyes: the first structure found by it is the cornea , a thin protective and transparent layer, filled with a liquid called aqueous humor , whose refractive index is similar to that of water, refracting light. and focusing it on the lens : a kind of thin , biconvex lens responsible for projecting all the light on a small point at the back of the retina , the optic nerve.

Refraction of light in water (blue background) and in air (white background)

In this way, when we are submerged, our vision becomes hyperopic , making it difficult to see nearby objects, as the light rays cross behind the retina.

One of the simplest solutions to implement to see more easily underwater is by introducing a layer of air between the cornea and the water with diving mask .

curious fact

Nearsighted people can see better underwater than others because the nearsighted eye is elongated in the horizontal direction, making the point of intersection of light rays closer to the optic nerve.