Why Is Everything Blurry Underwater in a Pool, While Fish See Clearly?
― A human eye can only focus properly with air in front of it
When you open your eyes underwater, nothing in the eye itself breaks. What you lose is the "boundary with air" on the eye's outermost surface. A human eye leaves about two-thirds of its focusing to that boundary. A fish's eye never counted on it in the first place.
A key has dropped to the bottom of the pool. You take a breath and open your eyes underwater to grab it. Your eyes are definitely open, but all you see is a blurry blob of colour.
Then you put on goggles, and the same water suddenly looks sharp. So the water was never cloudy.
In that same water, a fish is happily picking out tiny bits of food. What does a fish's eye have that ours doesn't?
There are two main reasons
The part of the eye that bends incoming light most is the cornea at the front, not the lens behind it. But the cornea can bend light only when air is on its outer side.
The cornea is mostly water. When the outside is water too, light sees no boundary at all and passes straight through.
Put the other way round, an eye built for life underwater, like a fish's, has to bend light by some means other than the cornea. Let's take it step by step.
Where does the bending happen?
Light changes direction only when it crosses a boundary between two different materials at an angle. The number that says how strongly a material bends light is called the refractive index. Air is about 1.00 and water is about 1.33. The bigger the difference between the two numbers, the more the light bends.
The refractive index of the human cornea is about 1.38. That is 0.38 more than air's 1.00, and the cornea is also a curved surface bulging forward. So light arriving from outside is bent sharply inward at the corneal surface. This one step supplies about two-thirds of the focusing power the eye needs.
The remaining third comes from the lens deeper inside. It is the adjuster, changing its thickness when you look at something close. So the human eye splits the job: one part does the heavy bending, the other does the fine-tuning.
What happens when you go underwater?
Nearly 80 percent of the cornea's weight is water. So its refractive index is close to water's, only about 1.38. The difference from water's 1.33 is just 0.05.
To light, a difference this small is almost the same as no boundary at all. The curved surface no longer does anything, and light enters the eye almost straight. The lens alone is not strong enough, so the light comes to a focus far behind the retina instead of on it. The right side of Figure 1 shows this.
Goggles work not because they contain a prescription lens. They simply rebuild a pocket of air in front of your eyes, so the outside of the cornea touches air again. Give the eye its lost boundary back, and it sees as usual without any change to the eye itself.
How did fish solve it?
A fish's cornea also has water on its outer side. So bending light at the cornea is not an option for a fish, and it hands the whole job to the lens.
That is why a fish's lens is not flat like ours but almost a perfect sphere. It is so hard you can pinch it between your fingers, and taken out of the eye it looks like a tiny glass marble. It is also built so that the refractive index rises towards the centre, which lets a single ball focus light to one point without colour smearing.
Focusing works differently too. We change the thickness of the lens, but a fish's lens is hard and cannot change shape. Instead, a small muscle moves the whole lens forward and back, much as you extend a camera lens. The right side of Figure 2 sets the two methods side by side.
Penguins and cormorants spot prey both in air and underwater. These birds have a gently curved cornea and a lens that can change shape a lot, so they are thought to make up for the shortfall underwater by themselves. Sea lions and seals take another route: their cornea is almost flat, giving them eyes tuned for water from the start. The price is thought to be slightly short-sighted vision on land.
If you go into the water wearing contact lenses, the lenses can wash away. Worse, microorganisms in the water can get trapped between the lens and the eye. This is said to occasionally lead to a hard-to-treat corneal infection, and Japanese eye-care societies and manufacturers advise taking lenses out for swimming or using prescription goggles.
Summary
Things look blurry underwater not because the eye loses to the water. It is because the human eye was built on the assumption that air is one of its parts. Fish did not count on that part. They chose a different answer: a hard, spherical lens. The word "eye" is the same, but the blueprints inside are different.
A human eye is complete only together with air.
A fish's eye is complete together with water from the start.
You can see light bending at a boundary in an even more familiar form in Why does a straw in water look bent? Seeing poorly in the dark is a different mechanism. Have a look at Why can't we see anything for a while when we step from bright light into a dark room? too.
- Fill a washbowl with water, put your face in, and look with bare eyes at a patterned towel on the bottom. Then put on goggles and look at the same thing. The water is the same, but the view changes.
- Put a small picture at the bottom of an empty cup and look down at it. Now pour in water gently. The picture seems to rise slightly. That is proof that the boundary between water and air bends light.
- If you buy a whole fish, look at its eye. Behind the dark part sits a hard, clear ball. You can see how different its shape is from the lens of a human eye.
If you try the face-in-water test, use only a shallow washbowl and do it with an adult watching. Stop at once if your eyes hurt, and rinse them with tap water.
For those who want more ― terms, formulas and links to textbooksShows which level each topic sits at, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school Physics Basics and Physics (waves and light)
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university-level courses (physiological optics, comparative ophthalmology)
- ResearchNot taught even at university as settled fact; questions researchers are studying now
Junior highTerms: this phenomenon has names
- Refractive index: a number showing how much a material slows light. With a vacuum as 1, air is about 1.00, water about 1.33 and the cornea about 1.38.
- Refractive power: how strongly a lens or curved surface bends light. Its unit is the dioptre, the reciprocal of the focal length measured in metres.
- Accommodation: changing focus to suit the viewing distance. Humans change the thickness of the lens, and many fish move the lens forward and back.
Junior highHigh schoolChecking with a formula: how many dioptres does the eye lose in water?
The subject of this article is how much power the cornea loses underwater. Let's work out that amount from the formula for refraction at a spherical surface.
| In symbols | P ＝ ( N₂ － N₁ ) ÷ R |
| In words | Refractive power ＝ (index behind － index in front) ÷ radius of curvature |
| Where it comes from | It applies the rule for how light bends at a boundary (the law of refraction) to light travelling close to the eye's axis. It is called the spherical refraction formula. If the indices on both sides of the boundary are equal, the subtraction on the right gives zero, and the bending power disappears. |
| Refractive index of the cornea | about 1.376 |
| Refractive index of air / water | about 1.000 / about 1.333 |
| Radius of curvature of the corneal surface (symbol R, in metres) | about 0.0078 (7.8 millimetres) |
| Refractive power the whole eye needs | about 60 dioptres |
| Gap from air | 1.376 － 1.000 ＝ 0.376 |
| Power of the cornea in air | 0.376 ÷ 0.0078 ≒ 48 |
| Gap from water | 1.376 － 1.333 ＝ 0.043 |
| Power of the cornea in water | 0.043 ÷ 0.0078 ≒ 5.5 |
| Power lost in water | 48 － 5.5 ＝ 42.5 |
| How many reading glasses (2.5)? | 42.5 ÷ 2.5 ＝ 17 |
Of the roughly 60 dioptres the whole eye needs, 42.5 disappear. Reading glasses are about 2.5 dioptres, so this is the power of 17 pairs vanishing in an instant. No wonder everything is blurry.
High schoolHigh school+How strong is a fish's ball lens?
High schoolA fish's lens is a sphere, so once the radius is set, the focal length is nearly set too. It has long been known that the focal length is about 2.55 times the radius, a relation called the Matthiessen ratio. For a lens with a radius of 3 millimetres, or 0.003 metres, 2.55 × 0.003 ＝ 0.00765 is the focal length in metres. Its power in water is 1.333 ÷ 0.00765 ≒ 174, about three times that of a whole human eye. It takes on the cornea's share as well.
High school+If you did this with an ordinary glass ball, light passing near the edge would focus closer than light through the centre, and the image would smear. This is called spherical aberration. A fish's lens is a graded-index lens, with the index rising towards the centre, and this nearly cancels the aberration. The decisive difference from a uniform glass ball is that light follows gently curving paths inside the lens.
UniversityModel eyes and underwater vision in comparative ophthalmology
In ophthalmology, the Gullstrand model eye, which replaces the cornea and lens with a few spherical surfaces, is used to treat the optics of the eye as a whole. Swapping the outside medium from air to water in this model reproduces this article's calculation directly and shows the eye becoming long-sighted by 40 dioptres or more. The design of graded-index lenses, and how animals that see in both water and air accommodate, are major topics in comparative ophthalmology.
📖 Derivations and further reading: Refractive power (Japanese Wikipedia) ／ Lens (Japanese Wikipedia)
ResearchWhat is still not clear
- How is the gradient made? The gradient of refractive index in a fish's lens is thought to be set by how concentrated a protein called crystallin is. But how the shape of the gradient is kept as the body grows is not yet fully explained.
- How well can humans see with training? There are reports that children of sea-dwelling peoples showed high visual acuity underwater with bare eyes, and squeezing the pupil small is thought to help. How far training can reproduce this is still debated.
- The limits of eyes that see in both. How much of the shortfall animals with sharp vision in both air and water make up by accommodation varies widely between species in the measurements, and no unified explanation has been reached.
So the content of this article is also "an explanation as far as we know now." The optics of the eye is an old field, but the fine details of each animal's design are still being studied.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: how light travels / structure of the eye | Light bending at a boundary; roles of the lens and retina |
| High school | Physics Basics and Physics (waves and light, law of refraction) | Spherical refraction formula and refractive power calculation |
| High school+ | Advanced physics (lens aberration) | Spherical aberration and graded-index lenses |
| University | Physiological optics / comparative ophthalmology | Gullstrand model eye, analysis of underwater vision |
| Research | Visual physiology / developmental biology | How the refractive index gradient is made and kept |
| ― | Links to daily life | Why goggles work, why to remove contact lenses in water |
- Refractive power ― Japanese Wikipedia (屈折力) (definition of refractive power and the dioptre)
- Lens ― Japanese Wikipedia (水晶体) (structure and accommodation of the lens)
- Standard values for the refractive power of the cornea and lens, based on the Gullstrand model eye. Widely given in ophthalmology and physiological optics textbooks.
- The Matthiessen ratio (the relation that a fish's spherical lens has a focal length of about 2.5 times its radius). It comes from a 19th-century report by L. Matthiessen and is still cited in reviews of comparative ophthalmology.
- Cautions on swimming with contact lenses in. Based on public guidance from Japanese ophthalmology societies and manufacturers.
※This article is a science explainer for general readers. The figures given are rough guides to help you understand how things work. For eye problems or how to handle contact lenses, follow the advice of an eye doctor or the manufacturer's instructions.