What if your eyes were underwater?

Your eyes make pictures of the world. To make a sharp picture, they bend light.

Under the water, things look fuzzy. What changes when your eyes are underwater?

Open your eyes in a swimming pool and everything looks blurry. But the water is clear, and your eyes work perfectly well in air. So what's going on?

The answer is about bending light. Your eye focuses light by bending it, and how much light bends depends on what it's passing from and into. In this lab, Olly shines a beam of light into a material like the inside of your eye, first from air, then from water.

Image formation in the eye depends on refraction, the bending of light at boundaries between materials of different refractive index. The National Eye Institute describes the cornea, the clear dome at the front of the eye, as bending light to help the eye focus, with the lens working alongside it.

This lab uses Snell's law to ask a precise question: how much does a beam bend entering an eye-like material from air, and how much from water? The answer explains why human vision is blurred underwater.

In air, light bends a lot when it goes into your eye.

That bend helps make a sharp picture inside your eye.

In the model, a beam comes down through air at 40 degrees and enters a material with a refractive index of 1.333, about the same as the watery inside of your eye. The beam bends by 11.2 degrees at the surface.

OpenStax explains that the biggest bend in your eye happens right at the front, at the cornea, because light slows down a lot going from air into it. The cornea provides about two-thirds of the eye's focusing power; the lens behind it adds the rest.

With n₁ = 1.000 (air), n₂ = 1.333 and an incidence angle of 40°, Snell's law gives a refracted angle of 28.8°: a bend of 11.2°, with 2.5% of the light reflected. Using the cornea's index of 1.38 (OpenStax Table 26.1), the bend is 12.2°.

OpenStax notes that the largest change in refractive index, and the largest bending, occurs at the cornea rather than the lens, and that the cornea provides about two-thirds of the eye's power because light's speed changes considerably from air into the cornea.

Now the beam comes through water instead of air.

It hardly bends at all! So your eye can't make a sharp picture.

Now change the top material to water, 1.333. The beam goes from water into a material just like water, so it doesn't bend at all: the bend readout shows 0.0 degrees.

The real cornea is a little different from water, about 1.38 instead of 1.33, so in real life there is still a tiny bend, about 1.6 degrees for this beam instead of 12.2 from air. Most of the cornea's focusing is gone. Diver magazine explains that the picture would then only come into focus behind the retina, at the back of the eye, so everything looks blurry.

Setting n₁ = 1.333 to match n₂ removes refraction entirely: the refracted angle equals the incident angle (40°) and the reflectance is 0%. With the cornea's 1.38, water-to-cornea refraction at 40° gives a bend of only 1.6°, against 12.2° from air.

A 2003 study by Gislén and colleagues in Current Biology states the consequence: in air the curved corneal surface accounts for two-thirds of the eye's refractive power, and this is lost when air is replaced by water, so the human eye is not focused underwater.

Light bends when it goes from one material into a different one.

Water and the front of your eye are almost the same. So the light barely bends.

Light travels at different speeds in different materials. Each material's refractive index tells you how much it slows light down. When light crosses into a material where its speed is different, it bends. The bigger the difference, the bigger the bend.

Air and the cornea are very different, so light bends a lot. But water and the cornea slow light by almost the same amount, 1.33 and 1.38. With so little difference, there's hardly any bend, and scientists who study underwater vision note that the human eye is left out of focus.

Snell's law, n₁ sin θ₁ = n₂ sin θ₂, makes the bend depend on the index ratio n₂ ÷ n₁: 1.38 from air, but only 1.38 ÷ 1.33 ≈ 1.04 from water. For a single curved surface, the imaging equation n₁ ÷ d_o + n₂ ÷ d_i = (n₂ − n₁) ÷ R (OpenStax University Physics 3, §2.3) ties its focusing to the index difference: about 0.38 from air into the cornea, only 0.05 from water, so the cornea's contribution collapses underwater.

The model's limits: it treats one flat boundary, a single wavelength and pure ray optics. A real cornea is curved and sits in front of the aqueous humor (1.34) and lens (1.41 on average), so the model shows the bending at one surface, not a complete eye or how sharp the final image would be.

Goggles or a mask keep a little air in front of your eyes.

Then light bends at your eye again, and you can see clearly!

Divers solve the problem with a mask. Writing in Diver magazine's diving medicine section, Dr. David Sawatzky explains that a mask puts a pocket of air in front of the cornea, so light crosses from air into the eye again and focuses normally.

Some children do remarkably well without masks. Scientists studied children of the Moken, who live by the sea in Southeast Asia and collect food from the sea floor. Underwater, the Moken children saw more than twice as sharply as European children, by making their pupils very small and focusing their lenses as hard as human eyes can.

Diver magazine's Dr. David Sawatzky explains that a dive mask creates an airspace in front of the cornea, restoring normal air-to-cornea refraction. Light leaving the mask's glass into that air spreads out a little, so objects look about 25% larger: a 40 cm fish appears about 50 cm long.

Gislén and colleagues (2003) measured underwater acuity in Moken children at 6.06 cycles per degree, more than twice the 2.95 of European children. The Moken children constricted their pupils to 1.96 mm (against 2.50 mm) and accommodated to about 15–16 diopters, the known limit of human performance, a response absent in the European children.