Why Do Thread-Like Shadows Float in Front of Your Eyes When You Look at a Blue Sky?
― What You See Is a "Shadow" Cast on the Retina by the Jelly Inside Your Eye
Look up at a clear sky and you may see threads or small rings drifting through your vision. Chase them and they slip away. They are not floating in the air outside. They are inside your eye. And what you see is not the speck itself, but its shadow. There is a good reason, based on light, why they show up sharply only in bright places.
On a clear autumn day, you are hanging laundry on the balcony and look up at the sky. At the edge of your vision, a translucent, thread-like shape drifts slowly past.
You turn your eyes to look at it, and it slides away sideways. When you hold your gaze still, it sinks slowly, a moment late.
But back indoors, in dim light, you look for it and can hardly see it at all. Where did it go?
There Are Only Two Reasons
The inside of your eyeball is filled with clear jelly. Clumps of fine fibres in it block some light and cast a shadow on the retina at the back.
In bright light, your pupil narrows. The smaller the opening for light, the less blurred the edge of the shadow, and the darker it is. In dim light, the shadow smears out and vanishes.
So those threads are "always there", but whether you see them depends on brightness. Let's go through it step by step.
What Is the "Jelly" Inside Your Eye?
Most of the inside of your eyeball is a clear jelly called the vitreous. It is mostly water, with fine fibres stretched through it like a net to hold its shape. When you are young, this net is even and lets light pass straight through.
With age, though, the jelly slowly becomes more watery, and the fibres gather together. These bunched fibres and small clumps block a little light. People with strong short-sightedness are said to be more likely to see this change early in life.
The important point is that you are not seeing the cloudy specks themselves. Your retina cannot directly see an object lying in front of it. What the retina receives is the "shadow" made when the specks block light.
Why Are They Sharp Only in Bright Places?
Look at Figure 1. The top row shows a small pupil in bright light, and the bottom row shows a large pupil in the dark. Light entering through the pupil on the left is blocked by a speck on the way and casts a shadow on the retina on the right.
The shadow blurs because light comes in from many directions, as many as the width of the pupil allows. Hold your hand under a single bulb and the shadow is sharp. Hold it in the light from a big window and the shadow is soft. The same thing happens inside your eye.
Under a bright sky, your pupil narrows to about 2mm. The blur of the shadow is then small, so even a thin thread casts a dark shadow. A blue sky or a white wall is also evenly bright all over, so only the shadow stands out. In a dark room, your pupil opens to about 6mm. The shadow of the same speck smears out three times wider and melts into the background.
Why Do They Run Away When You Chase Them?
The shadow forms at a fixed spot on the retina. So when you turn your eyes toward it, the shadow moves along with the retina. Try to bring it to the middle of what you want to look at, and each time your eye moves, the shadow shifts by the same amount.
The specks also float in the jelly. Even if you stop your eyes suddenly, the jelly inside keeps swaying a little longer. That is why, after you stop moving your eyes, the thread drifts a little and then sinks slowly.
When you gaze idly at a blue sky, you may see tiny points of light darting around in wiggly paths. This is a different phenomenon from the threads. It is thought to be white blood cells moving through the thin blood vessels of the retina. Only the white cells' paths, which let blue light through more easily, flash brightly for a moment. That the dots speed up in time with your pulse is also taken as evidence.
A few threads you have had for a long time are usually nothing to worry about. But some changes should not be ignored: a sudden increase in number, a burst of black dots like soot, flashes like lightning in the dark, or a curtain-like shadow closing in from the edge of your vision. These can be signs that the retina has a tear or is starting to detach. The sooner it is treated, the better the chance of protecting your sight. Do not wait and see. Go to an eye doctor.
Summary
The threads drifting before your eyes are shadows cast on the retina by cloudy specks in the jelly inside your eye. In bright places, your pupil narrows and the edges of the shadow stay sharp, so you see them clearly. In the dark, your pupil opens, the shadow smears out, and they disappear.
Those threads are always in your eye.
What decides whether you see them is the size of your pupil.
To learn how far your pupil opens in the dark, read "Why Can't You See Anything for a While When You Walk from Bright Light into a Dark Room?" And for why your eyes lose focus underwater, read "Why Is Your Vision Blurry When You Open Your Eyes in a Pool, but Fish See Clearly?"
- With a toothpick, poke a small hole about 1mm across in a piece of card or a business card.
- Aim at a bright white wall, or at a part of the clear sky well away from the sun, and look through the hole held right in front of your eye.
- The hole is an even smaller opening than your usual pupil, so the shadows of threads and rings should look sharper than usual. Move your eye once and then hold it still, and you can watch the shadows drift slowly.
*Never look at the sun itself. Even when looking through the hole, point it at a part of the sky well away from the sun.
For Those Who Want to Know More ― Terms, Formulas, and Links to TextbooksWe show clearly what level each topic is, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics or biology
- High school+Advanced high school material, or textbook sidebar content
- UniversityNot taught in high school; university specialist subjects (physiological optics, ophthalmology)
- ResearchNot taught even at university as settled fact; topics researchers are investigating now
Junior highTerms: This Phenomenon Has a Name
- Floaters (myodesopsia): The name for seeing mosquitoes or threads flying in front of your eyes. Most cases are natural changes with age.
- Vitreous: The clear jelly that fills the eyeball. It is mostly water, with fine fibres supporting its shape.
- Umbra and penumbra: The umbra is the core of the shadow where no light reaches. The penumbra is the blurry-edged part where only some of the light reaches.
Junior highHigh schoolChecking with a Formula: How Blurry Is the Edge of the Shadow?
Light from the top and bottom edges of the pupil grazes the edges of the speck and reaches the retina. The similar triangles formed by these two lines give the width of the blur at the edge of the shadow.
| In symbols | w ＝ D × d ÷ L |
| In words | Shadow blur width ＝ pupil diameter × (distance from speck to retina ÷ distance from pupil to retina) |
| Where it comes from | Light travels in straight lines. So the rays leaving the edges of the pupil and passing the edges of the speck form one triangle with the pupil as its base and another with the blur width as its base. The formula comes from these two triangles being similar. |
| w | Blur width of the shadow on the retina (width of the penumbra). Unit: mm |
| D | Pupil diameter. Unit: mm |
| d | Distance from speck to retina. Unit: mm |
| L | Distance from pupil to retina. Unit: mm |
| Distance from pupil to retina L | About 20mm |
| Pupil diameter D in bright light | About 2mm |
| Pupil diameter D in the dark | About 6mm |
| Distance from speck to retina d (as an example) | 2mm |
| Thickness of a thread-like speck (as an example) | 0.2mm |
| Bright light: pupil diameter × speck distance | 2 × 2 ＝ 4 |
| Blur width in bright light (mm) | 4 ÷ 20 ＝ 0.2 |
| Dark: pupil diameter × speck distance | 6 × 2 ＝ 12 |
| Blur width in the dark (mm) | 12 ÷ 20 ＝ 0.6 |
| How many times larger is the blur in the dark than in bright light? | 0.6 ÷ 0.2 ＝ 3 |
| Looking through a 1mm hole: hole diameter × speck distance | 1 × 2 ＝ 2 |
| Blur width when looking through a 1mm hole (mm) | 2 ÷ 20 ＝ 0.1 |
In bright light, the blur width stays about the same as the thickness of the thread, so a dark core remains in the shadow. In the dark, the blur width spreads to three times the thickness of the thread. The darkness of the shadow is thinned out over a wide area and can hardly be told apart from the background. If you replace the pupil with a 1mm hole, the blur width shrinks to 0.1mm and the shadow becomes even sharper. The formula also shows that a speck closer to the retina has a smaller d and is seen more sharply.
High schoolHigh school+Straight-Line Light and the "Self-Shadow" Inside the Eye
High schoolThe core and blur of the shadow can be explained by light travelling in straight lines alone. It is the same geometry that gives the Moon's shadow an umbra and penumbra in a solar eclipse. The "pupil" plays the part of the light source, the "speck" is the object blocking light, and the "retina" is the screen that shows the shadow.
High school+Phenomena in which structures inside the eye appear in your own vision are together called entoptic phenomena. Besides the shadows of threads, the shadows of retinal blood vessels and the flow of white blood cells are also entoptic phenomena. The shadows of retinal blood vessels are normally invisible because they always fall in the same place, and the brain is thought to erase them from what you see.
UniversityPosterior Vitreous Detachment and Shadows Seen Through Physiological Optics
With age, the vitreous jelly liquefies and the fibres gather. Eventually the back of the jelly peels away from the retina, which is called posterior vitreous detachment, and it is said to be seen mostly in people in their 50s and 60s. When this happens, a part that was firmly attached to the retina can be pulled and torn, causing a retinal tear, and if left alone the tear can progress to retinal detachment. This is why people are advised to have a dilated fundus examination if floaters suddenly increase. In physiological optics, a schematic eye, which replaces the eye with average dimensions, is used, treating the pupil as an aperture and the speck as an obstruction to handle the umbra and penumbra of the shadow.
📖 Derivations and further reading: Floaters (Wikipedia, in Japanese) ／ Umbra and penumbra (Wikipedia, in Japanese)
ResearchWhat Is Still Not Clear
- Why do some people mind them and others don't? Even with similar specks, some people are badly bothered while others get used to them quickly. It is thought to depend on whether the brain can treat the shadow as "information to ignore", but the mechanism is still being studied.
- How well does laser treatment to break up the specks work? There is a method that fires a laser at the specks to make them smaller, but opinion is still divided on how much it helps and for whom.
- What triggers the jelly to liquefy. We know it is linked to age and strong short-sightedness, but why the fibre net starts to break down, and in what order at the molecular level, is not fully understood.
So the content of this article, too, is "an explanation of what is known so far". The geometry of the shadow is clear, but how the brain deals with it still holds some mysteries.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "Properties of light", "Structure of the eye" | Straight-line light, roles of pupil and retina |
| High school | Physics: "Straight-line light"; Maths: "Similar figures" | Formula for shadow blur width |
| High school+ | Biology: advanced "Receptors and nerves" | Entoptic phenomena, invisible blood vessel shadows |
| University | Physiological optics, ophthalmology | Posterior vitreous detachment, retinal tear, schematic eye |
| Research | Visual neuroscience; retina and vitreous research | Individual differences in getting used to floaters, evaluating treatment |
| ― | Link to daily life | Deciding to see an eye doctor if they suddenly increase |
- Wikipedia, "Floaters" (飛蚊症, in Japanese)
- Wikipedia, "Retinal detachment" (網膜剥離, in Japanese)
- Wikipedia, "Umbra" (本影, in Japanese)
- Japanese Ophthalmological Society (日本眼科学会), "Eye diseases: Floaters" (飛蚊症; general-reader explainer)
- Sebag, J. (ed.) "Vitreous: in Health and Disease", Springer
*This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. If you notice changes in your vision, do not diagnose yourself; see an eye doctor.