Why do anglers wear polarized sunglasses?
― The glare on water is "light shaking sideways"
The longer someone has been fishing, the less likely they are to go without a tinted pair of "polarized sunglasses." Put them on, and the glare on the water disappears, revealing the stones and fish beneath the surface. Ordinary sunglasses don't do this. Light reflected off water actually has a special property: its shaking lines up in one direction. Polarized lenses work by picking out only that direction of light and blocking it.
A river on a sunny day. Even leaning over a bridge to look down at the water, the sky is reflected white on the surface, and you can barely see anything below.
But the person fishing next to you points and says, "There's a fish in the shadow of that rock." They're wearing polarized sunglasses.
Borrow them and put them on, and the reflection just melts away — the stones on the riverbed come into sharp view. Tilt your head sideways, and the glare comes right back.
Two reasons the glare alone disappears
Light is a wave, shaking side to side as it travels. Sunlight and skylight are a mixture of light shaking in every direction. But when it reflects off water, mostly the "sideways shaking" parallel to the surface survives.
A polarized lens has an extremely fine structure, like vertical stripes, that lets through only light shaking vertically. The sideways-aligned reflection gets blocked, while the mixed-direction light coming from underwater gets through at about half strength.
Ordinary sunglasses dim all light by the same amount. Both the reflection and the fish's shadow get darker together, so the difference in how visible they are doesn't change. Polarized sunglasses are different: they can "choose" to cut just the reflection.
At the water's surface, light gets "sorted"
Look at the left side of Figure 1. When light from the sky hits the water's surface, some of it bounces back and the rest enters the water. How easily it bounces back depends on which way it's shaking. Sideways shaking parallel to the surface bounces back easily; shaking at right angles to that tends to enter the water instead.
So the reflected light ends up mostly sideways-shaking. Meanwhile, light that scatters off a fish or a stone underwater and comes back up has its shaking directions all mixed up. As in the right side of Figure 1, if you place a lens that only lets vertical shaking through, the reflection is almost entirely blocked while the light from underwater gets through.
The sweet spot: about 37 degrees above the water
How strongly the light aligns sideways depends on your viewing angle. Look at Figure 2. Looking straight down, only about 2% of the light reflects back, and it isn't aligned either. The closer you look to skimming along the surface, the stronger the reflection gets, and the more the glare dazzles you.
Somewhere in between is an angle where the reflected light becomes almost purely sideways-shaking. For water, that's about 37 degrees above the surface. That's roughly the angle of standing on the bank and looking at the water a little way out. So when you peer at the water just off the bank, that's exactly where polarized lenses work best.
Conversely, when looking nearly straight down, polarized lenses have little effect. And when looking almost level with the surface, the reflection of vertical shaking increases too, so the glare never fully disappears.
Because rotating the lens 90 degrees swaps the direction it lets through from "vertical" to "horizontal." That's how you can tell whether sunglasses are polarized. Many phone and car displays emit light whose shaking is already aligned in one direction. That's why, viewed through polarized sunglasses, a screen can look dark depending on the angle.
Some aquatic insects are thought to use sideways-aligned light as a landmark for finding water surfaces. Glossy black car roofs and roads also reflect light that aligns sideways. As a result, there are reports of insects mistaking them for water and laying eggs on them.
Summary
Light reflected off water shakes mostly sideways. Polarized sunglasses only let vertical shaking through, so they selectively block the reflection while letting light from underwater pass. The alignment is strongest at about 37 degrees above the surface. The angler's experience of "polarized glasses let me see the fish" turns out to be a direct consequence of light's wave nature.
A reflection isn't just brightness — it's "light aligned in one direction."
Because it's aligned, it can be selectively removed.
The same window glass that's see-through by day and a mirror at night is explained in "Why does window glass turn into a mirror at night?". Skylight, too, is actually slightly aligned in its shaking. You can read why in "Why is the sky blue, but sunsets red?".
- On a sunny day, find a surface reflecting light — a pond, a puddle, a wet road, or a car window.
- Look through polarized sunglasses, then slowly tilt your head 90 degrees. Notice the reflection getting stronger and weaker.
- Try changing your viewing angle. Compare how the reflection disappears when looking nearly straight down versus looking at the water a little way out.
Without polarized sunglasses, you can try looking at a phone screen at an angle through the camera of another phone instead. When observing near water, watch your footing on wet rocks and don't stray too far from the bank.
Want to know more? ― Terms, formulas, and textbook connectionsLabels show whether each part is middle-school science or university-level, so you know what you're getting into
- MSCovered in middle-school science
- HSCovered in high-school physics
- HS+Advanced high-school content, or textbook sidebar material
- UnivNot covered in high school — university-level electromagnetism/optics
- ResearchNot yet settled textbook fact even at university — an active research topic
MSTerms: this phenomenon has a name
- Polarization: light whose shaking direction is aligned. Sunlight or lamplight, with directions all mixed up, is called "unpolarized light" or "natural light."
- Refractive index: a number showing how much slower light travels inside a material. For water, it's about 1.33.
- Brewster's angle (polarizing angle): the angle of incidence at which reflected light becomes shaking in only one direction. For water, that's about 53 degrees.
MSHSWorking it out: finding the angle of strongest alignment
We find the angle at which the reflection becomes purely sideways-shaking (Brewster's angle) from water's refractive index. For comparison, we also work out the reflected fraction when viewed straight down. Here's what the symbols mean:
| θB | Brewster's angle. Measured from a line perpendicular to the water's surface (in degrees) |
| n1 | Refractive index of the medium light comes from (air). About 1.00 |
| n2 | Refractive index of the medium light enters (water). About 1.33 |
| R | Fraction of light reflected when hitting straight down |
| In symbols | tan θB ＝ n2 ÷ n1 ／ Looking straight down: R ＝ ( (n2 − n1) ÷ (n2 + n1) )² |
| In words | "Tangent of the angle ＝ water's refractive index ÷ air's refractive index." At this angle, the reflected shaking is only in the direction parallel to the surface |
| Where it comes from | When the reflected ray and the ray entering the water end up exactly at right angles to each other, vertically-shaking light can't radiate in the reflected direction. Combine this right-angle condition with the law of refraction (Snell's law) and you get Brewster's law |
| Refractive index of water | About 1.33 |
| Refractive index of air | About 1.00 |
| Reflection when skimming the surface (10 degrees above water) | Calculated at about 35% |
| Ratio of refractive indices (＝ tan θB) | 1.33 ÷ 1.00 ＝ 1.33 |
| Angle whose tangent is 1.33 | About 53 degrees |
| Angle measured from the surface | 90 − 53 ＝ 37 degrees |
| Straight down: difference of refractive indices | 1.33 − 1.00 ＝ 0.33 |
| Straight down: sum of refractive indices | 1.33 + 1.00 ＝ 2.33 |
| Straight down: difference ÷ sum | 0.33 ÷ 2.33 ≒ 0.14 |
| Straight down: squared, gives reflected fraction R | 0.14 × 0.14 ≒ 0.02 |
| How much brighter is skimming the surface than straight down | 35 ÷ 2 ＝ 17.5 times |
Looking at about 37 degrees above the water, the reflection becomes almost purely sideways-shaking, and a polarized lens blocks nearly all of it. Looking straight down, only about 2% of the light reflects back. Skimming the surface, that jumps to roughly 17 times as much, making it dazzling.
HSHS+Light is a "transverse wave" — that's why direction can be selected
HSLight is a transverse wave, shaking at right angles to the direction it travels. Sound, a longitudinal wave, has no freedom to choose a "shaking direction," so it has no polarization. If you stack two polarizing filters and rotate one, the view goes completely dark at 90 degrees. This appears in textbooks as proof that light is a transverse wave.
HS+The intensity of light passing through a polarizing filter is proportional to the square of the cosine of the angle between the filter's transmission direction and the light's shaking direction (Malus's law). Tilt your head 45 degrees, and half of the sideways-aligned reflection gets through.
UnivThe Fresnel equations: reflectance by shaking direction
The reflectance at an interface, split into shaking perpendicular to the plane of incidence (s-polarization) and parallel to it (p-polarization), is given by the Fresnel equations. The two curves in Figure 2 were calculated from these equations. The angle at which the p-polarization reflectance drops to zero is Brewster's angle, derived from how the electric and magnetic fields connect across the boundary (the boundary conditions). This is why light reflected off water is biased toward s-polarization — shaking parallel to the surface.
📖 For the derivation and further reading: Fresnel equations (Japanese Wikipedia) ／ Brewster's angle (Japanese Wikipedia)
ResearchWhat isn't fully understood yet
- How far do water-dwelling animals rely on polarization?Some aquatic insects are known to use polarization at the water's surface as a cue. But exactly how fish, squid, and octopuses use polarization underwater is still being studied species by species.
- Effects of artificial reflective surfaces on wildlife."Polarized light pollution" — black panels, glass, and roads mistakenly attracting insects — still has an effect size and countermeasure effectiveness that are under evaluation.
- How things look on a rippled surface.A real water surface has ripples that constantly change its tilt. How much a polarized lens improves underwater visibility in that case varies a lot by conditions and isn't captured by a simple formula.
In short, this article, too, describes things "as currently understood." Especially at real bodies of water, murkiness and sky brightness also change how visible things are by a lot.
Textbook connections (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science — reflection and refraction of light | Light splits into a reflected part and a part entering the water |
| HS | Physics — waves (transverse waves and polarization) | Polarized lenses only let vertical shaking through |
| HS+ | Physics — polarization of light (advanced) | Malus's law, calculating Brewster's angle |
| Univ | Electromagnetism/optics | Fresnel equations, s- and p-polarization |
| Research | Visual ecology / light-pollution research | How animals use polarization, artificial misidentification |
| ― | Everyday connections | Fishing, driving, glare on snow, polarizing filters in photography |
- Wikipedia, "Brewster's angle" (ブリュースター角)
- Wikipedia, "Fresnel equations" (フレネルの式)
- Wikipedia, "Polarization" (偏光)
- E. Hecht, "Optics", Pearson (chapters on polarization and reflection)
- G. Horváth et al., "Polarized light pollution: a new kind of ecological photopollution", Frontiers in Ecology and the Environment, 2009
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanism. Reflection fractions are calculated values assuming a flat, calm water surface. Please mind your footing and the weather when near water.