Why does a straw in water look bent?
― Light changes direction where it leaves the water
The straw isn't bent. What's bent is the path the light takes from the straw to your eye. And your eyes simply can't spot the kink.
You put a straw at an angle into a glass of juice. Looking down from above, the straw seems to kink sharply right at the surface of the water.
A child asks, "why is it bent?" You pull the straw out to show them. It's perfectly straight. You put it back in, and it looks bent again.
It's easy to say "that's light refraction." But why would bending light make a straw look bent? Surprisingly few adults can explain that part.
Just two reasons
Light travels more slowly through water than through air. When it crosses that boundary at an angle, its direction bends.
Your brain has no way of knowing whether the light reaching your eye bent along the way. It assumes an object sits wherever a straight line traced back from that light would land.
Put these two together, and the part of the straw underwater appears "shallower than it really is." The part above water looks exactly where it is, so the position jumps at the surface — and the straw looks bent.
Why does light bend at the boundary?
The speed of light changes depending on what it's passing through. In water, it's thought to travel at about three-quarters of its speed in air.
Why would a change in speed also change direction? Picture a line of people holding hands, walking at an angle into a beach from a paved path. The ones who step onto the sand first slow down. The ones still on the pavement keep moving fast. The line ends up swinging to point more toward the sand.
Light waves behave the same way. Going from water into air is the reverse: the side that reaches the air first speeds up. So the light bends toward a direction that lies flatter against the water's surface.
Look at Figure 1. The solid line at bottom right is light leaving the tip of the straw. It changes direction at the surface and reaches the eye at top right.
Why does the brain get fooled?
All your eye can register is "which direction did this light arrive from." Light carries no record of where along its path it bent.
In everyday life, light travels through air in almost a straight line. So assuming "the object is wherever the incoming light points" is usually the right call. The brain applies this same handy rule even when looking into water.
Every point along the underwater part of the straw sends light that bends by the same amount at the surface. So that whole underwater section appears to shift upward together. The part above water doesn't shift, which is exactly why the "bend" appears right at the surface.
Viewed from the side of the glass, the straw can even look shifted left or right, as if it's been cut apart. That happens because the curved side of the glass acts like a lens — but it's the same underlying mechanism of light bending at a boundary.
Looking straight down, the bottom of the water is thought to appear at about three-quarters of its true depth. A spot that looks knee-deep might actually reach your waist. Don't trust how deep water looks near the shore.
Archerfish, which knock insects off overhanging leaves by spitting water at them, are reported to correct their aim for the amount light bends. Kingfishers and other birds that dive for fish are also thought to learn to compensate for the shift through experience.
Summary
A straw in water looks bent because light changes direction at the surface, while the brain assumes light always travelled in a straight line. As a result, only the underwater part appears to shift into shallower territory.
It isn't the straw that's bent — it's the path of the light.
Your eyes simply can't see where it turned.
Light bending at a boundary also explains why rainbows appear opposite the sun, why mirages appear, and why a magnifying glass can set paper on fire.
- Place a coin at the bottom of an opaque mug.
- Lower your head slowly until the coin just disappears behind the rim. Stop there without moving your head.
- Have someone else gently pour in water without disturbing the coin. The coin that was hidden will rise back into view above the rim.
The coin hasn't moved a millimetre. Light bending at the water's surface is now reaching your eye over the rim. It's exactly the same mechanism as the bent-looking straw.
Want to know more? ― Terms, formulas, and where this fits in textbooksLabels show whether a point is covered in middle school, high school, or university-level science
- MSCovered in middle-school science
- HSCovered in high-school physics
- HS+Advanced high-school material, or a textbook sidebar topic
- UnivNot covered in high school — university-level optics/electromagnetism
- ResearchNot yet settled even at university level — an active research question
MSTerminology: this phenomenon has a name
- Refraction: the change in direction light undergoes when crossing a boundary between materials with different optical properties.
- Refractive index: a number showing how much slower light travels in a material compared with a vacuum. Water is about 1.33; air is close to 1.
- Apparent depth: how deep the bottom of the water looks from outside. Viewed straight down, it's close to the true depth divided by the refractive index.
MSHSCheck with the formula: light's speed in water, and apparent depth
Using the refractive index, we can calculate how fast light travels in water, and how much shallower a pool floor looks.
| Speed of light in a vacuum (km/s) | about 300,000 |
| Refractive index of water | about 1.33 |
| True depth of the pool (cm) | 100 |
| Speed of light in water (km/s) | 300,000 ÷ 1.33 ≒ 225,564 |
| Apparent depth viewed from directly above (cm) | 100 ÷ 1.33 ≒ 75 |
| How much deeper than it looks (cm) | 100 － 75 ＝ 25 |
Water that's one metre deep looks about 75 cm from directly above. Viewed at an angle, the gap is even larger.
HSHS+The law that determines the bending angle
HSConsider the angle between the light ray and a line perpendicular to the boundary. "Refractive index × the sine of the angle on that side" is equal on both the air side and the water side. This is the law of refraction (Snell's law). Take light travelling in water at 30 degrees from the perpendicular.
| Sine of the 30-degree angle in water | 0.5 |
| Sine on the air side (multiply by water's refractive index) | 0.5 × 1.33 ＝ 0.665 |
| Angle on the air side | about 42 degrees |
Light travelling at 30 degrees in water flattens out to about 42 degrees once it leaves for the air. That's the bend shown in Figure 1.
HS+Conversely, light in water whose angle from the perpendicular is too large can't escape into the air at all — it bounces entirely back off the surface. This is called total internal reflection, and for water's refractive index it's said to occur past about 49 degrees. That's why, looking up from underwater, the scene outside the surface appears squeezed into a single round window.
UnivWhy does slowing down cause bending? The principle of least time
Light travelling from one point to another can be described as taking the path that takes the least time (more precisely, a path where nearby paths take almost exactly the same time). This is called Fermat's principle. It's the same logic as a lifeguard racing from the beach to someone in the sea: they run further on the fast sand before entering the slower water — and this logic yields the law of refraction. In electromagnetism, the same law also follows from the conditions that link electric and magnetic fields across a boundary.
ResearchWhat's still not fully understood
- How do animals compensate for the refraction shift? Archerfish and herons have been observed correcting for it, but whether this is innate or learned, and where in the brain the calculation happens, remains under study.
- How much can people learn to correct for the position of underwater objects? How well fishers and people who live near water adapt to the apparent shift hasn't been thoroughly studied.
- What "light slowing down" actually means. Light slowing in a material is explained as arising from electrons inside atoms being shaken and re-emitting light that overlaps with the original. How best to describe this in terms of light as particles is still debated in teaching circles.
In other words, even this article describes things "as currently understood." That light bends at a boundary is itself an extremely well-established fact, confirmed for over a thousand years.
Where this fits in textbooks (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Year 1 Science, "Refraction of Light" | Light changing direction at a boundary / the coin experiment |
| HS | Physics, "Refraction of Waves" | Refractive index and the law of refraction |
| HS+ | Physics, "Total Internal Reflection" | The round window seen looking up from underwater |
| Univ | Optics / Electromagnetism | The principle of least time |
| Research | Animal behaviour / vision science | How animals correct for the shift |
| ― | Everyday connection | Water is deeper than it looks |
- MEXT-certified middle school science textbook (Year 1), "Properties of Light" (文部科学省検定済 中学校理科教科書)
- E. Hecht, Optics, Pearson
- R. P. Feynman, QED: The Strange Theory of Light and Matter, Princeton University Press (Japanese edition: 『光と物質のふしぎな理論』, Iwanami Shoten)
- L. M. Dill (1977) Refraction and the spitting behavior of the archerfish (Toxotes chatareus). Behavioral Ecology and Sociobiology 2
- R. Rashed (1990) A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses. Isis 81
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding of the underlying mechanism. Near water, please follow local signage and the instructions of site managers or authorities.