Why Can't We See Air?
― It Isn't Invisible, It Just Has No "Boundary"
Has a child ever asked you, "Why can't we see air?" and left you lost for words? "Because it's too small" is only half right. The real reasons are that air doesn't absorb light, and that it has no "boundary" for light to bounce off. And in fact, under the right conditions, air can be seen quite well.
On a windy day, the air pushes against your face. Wave a fan and your hair sways. Your body knows perfectly well that air is there.
Yet when you look in front of you, there is nothing to see. The air in a room weighs tens of kilograms in total.
A clear glass of water, on the other hand, is easy to see. Water and glass are both transparent, so why is only air "invisible"?
There are only two reasons
Nitrogen and oxygen, which make up most of air, hardly absorb visible light. With no light taken out, there is no colour. The light simply passes straight through.
We can see transparent things because light bounces back a little, or bends, at their surface. Air is joined to more air, so it has no such boundary.
So "invisible" doesn't mean our eyes are slacking. Air simply sends almost no clues to our eyes. Let's take the two reasons in turn.
No colour, because light passes straight through
Things have colour because they swallow part of the light. A leaf, for example, absorbs red and blue light and bounces back the green that is left.
Nitrogen and oxygen do absorb light too. But the light they absorb is invisible, such as ultraviolet. Some of the infrared, which carries heat, is absorbed by water vapour and carbon dioxide in the air.
Only the visible light, which sits right between those two, passes almost entirely through air. That is why air has no colour.
Some people think this is no accident. Living eyes are thought to have evolved to make good use of the light that gets through air and water. What we call "visible light" is, to begin with, light that air does not easily absorb.
What we see is not the "thing" but its "boundary"
If lack of colour were the whole story, a glass cup would be invisible too. But we can see it. The reason is the second one: the "boundary".
When light reaches the boundary between two different materials, part of it bounces back. The rest changes direction a little as it goes on. The faint reflection of your face in a window is that bouncing. The warped rim of a glass is that bending.
How much light bounces back depends on how differently the two sides of the boundary "slow down" light. The number that describes this is called the refractive index. Try changing the difference with the slider in Figure 1.
Set Figure 1 to glass (refractive index about 1.5) and about 4% of the light bounces back. Those 4% are what tell us "there is glass here."
The refractive index of air, however, is about 1.0003, almost the same as empty space (exactly 1). Between one patch of air and the next there is simply no difference. With no boundary, there is no bouncing and no bending. No clue is ever produced for us to see.
Sink a glass rod into oil with about the same refractive index as glass, and the rod almost disappears. Once the difference at the boundary is gone, glass becomes "unseeable" just like air. You can try it in the activity below.
Even so, air can sometimes be "seen"
Air is not completely unseeable. It shows itself in two situations.
The first is when you pile up a lot of air. Air molecules scatter light sideways, just a tiny bit at a time, and blue light is scattered most. You can't notice it over one metre, but across 10 kilometres about a tenth of the light is scattered. Distant mountains look bluish and hazy because you are seeing the air between you and them. The blue of the whole sky works the same way (Why the sky is blue).
The second is when temperature differences appear in the air. Warm air has a slightly lower refractive index than cold air. The shimmering scenery above summer asphalt, called a heat haze, is light bent because this small difference acts as a "boundary." When the difference is larger and spread more evenly, you get a mirage.
Summary
We can't see air because it doesn't absorb visible light and has no boundary for light to bounce off. It has nothing to do with being small. That is why, when you pile up tens of kilometres of it, or create a boundary with a temperature difference, air becomes visible too.
To see something is to have light change direction at a "boundary."
Air, which has no boundary, shows itself only as the blue of distant mountains.
A related story of something "transparent yet looking white" is in Why is snow white when ice is clear? That one is an example with far too many boundaries.
- Pour cooking oil into a clear glass cup until it is about half full. A small heat-resistant glass container, a glass rod or a marble makes it easier to see.
- Slowly lower a small piece of glass into the oil and look from the side. Compare it with the same glass in water. In water it looks sharp; in oil its outline fades.
- On a clear day, compare distant mountains or buildings. The nearer ones should look darker, and the farther ones paler, bluish and hazy.
Refractive index differs a little between types of glass, so it may not vanish completely. Cooking oil is around 1.47 and ordinary glass around 1.5. Use oil from your kitchen, and take care not to break the glass.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle-school science to university-level courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics
- High school+Advanced high-school material, or shown in textbook sidebars
- UniversityUniversity courses not taught in high school (optics, electromagnetism, atmospheric radiation)
- ResearchTopics researchers are still investigating, not yet taught as settled
Middle schoolTerms: this phenomenon has names
- Reflection and refraction: when light reaches the boundary between different materials, part of it bounces back (reflection) and the rest changes direction and goes on (refraction). Taught in first-year middle-school science.
- Refractive index: a number showing how much slower light travels inside a material than in empty space. Air is about 1.0003, water about 1.33, glass about 1.5 and diamond about 2.42.
- Absorption: when a material takes in the energy of light. The absorbed colours go missing, which is how things get their colour.
Middle schoolHigh schoolCheck with a formula: how much light bounces off a boundary?
When light hits a boundary head-on, the share that bounces back depends only on the refractive indices of the two sides. Let's compare a glass–air boundary with an air–air one.
| In symbols | R ＝ ( (n₁ − n₂) ÷ (n₁ + n₂) )² |
| In words | Share reflected ＝ the square of "difference in refractive index ÷ sum of refractive indices" |
| Where it comes from | Light is a wave of electricity and magnetism. The formula follows from the condition that the electric and magnetic strengths of that wave must join up on both sides of the boundary (the Fresnel equations, limited to head-on incidence). |
Meaning of the symbols: R is the share of light reflected (no unit; 0.04 means 4%), and n₁ and n₂ are the refractive indices on either side of the boundary (no unit).
| Refractive index of glass | About 1.5 |
| Refractive index of air | About 1.0003 (differs from empty space by 0.0003) |
| Rough guide to scattering of green light in air near sea level | After about 85 km, it is said to fall to a little under 40% |
| Glass: difference in index | 1.5 − 1 ＝ 0.5 |
| Glass: sum of indices | 1.5 + 1 ＝ 2.5 |
| Difference ÷ sum | 0.5 ÷ 2.5 ＝ 0.2 |
| Square it: share reflected | 0.2 × 0.2 ＝ 0.04 |
| Air: difference ÷ sum (sum is almost 2) | 0.0003 ÷ 2 ＝ 0.00015 |
| Square it: share reflected | 0.00015 × 0.00015 ＝ 0.0000000225 |
| How many times more glass reflects than air | 0.04 ÷ 0.0000000225 ≒ 1780000 |
| Rough share scattered by 10 km of air | 10 ÷ 85 ≒ 0.12 |
A glass surface reflects 4% of the light. The boundary between air and empty space reflects only about 2 parts in 100 million. The difference is roughly 1.8 million times. Within real air there is not even an air–vacuum boundary, and the boundaries made by temperature differences are smaller still. On the other hand, passing through 10 km of air scatters about a tenth of the light sideways. The numbers confirm it: air "can't be seen at a boundary, but can be seen when you pile up enough of it."
High schoolHigh school+Why does light change direction at a "boundary"?
High schoolThe refractive index is a ratio of light speeds. Inside glass, light travels at about two-thirds of its speed in empty space. Refraction is the wave changing direction at a boundary where its speed changes, and the relation is described by Snell's law. A heat haze happens because air at different temperatures has slightly different refractive indices, so the path of light bends little by little.
High school+The strong scattering of blue light in air is called Rayleigh scattering. Its strength is inversely proportional to the fourth power of the wavelength. Blue (wavelength about 450 nanometres) is scattered roughly 4 to 6 times more strongly than red (about 700 nanometres). In painting, the blue haze of distant scenery has long been called "aerial perspective."
UniversityRefractive index is set by the electrons inside matter
When light passes through matter, its electric field shakes the electrons inside. The shaken electrons emit new light, which overlaps the original light and makes its apparent speed slower. This is what refractive index really is. The refractive index minus 1 is roughly proportional to the number density of molecules. Air is packed only about 1/800 as densely as water, which is why its refractive index difference is almost zero. At wavelengths far from the resonances where electrons absorb light (the ultraviolet region for nitrogen and oxygen), almost no absorption occurs and the material is transparent. This idea is studied in electromagnetism and optics as the Lorentz model and the theory of dispersion. The share reflected at a boundary is given by the Fresnel equations, derived from boundary conditions.
📖 Derivations and further reading: Fresnel equations (Wikipedia, in Japanese) ／ Rayleigh scattering (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Why eyes have their range of "visible light." The usual explanation is that the light that passes easily through air and water overlaps with the wavelengths where sunlight is strongest. But how much each factor mattered is still debated.
- Distant visibility and tiny particles in the air. Real air contains small particles such as water droplets and dust, and visibility changes greatly with them. The link between their amount and how things look is still being observed by region and season.
- The fine shape of heat-haze shimmer. The way temperature differences move as swirls is complex. In astronomy, correcting for this "atmospheric turbulence," which makes telescope images wobble, is an active area of research.
In other words, even this article is "an explanation within what is known today." That air is transparent is certain, but how far you can see also depends on things other than air.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, year 1: "Reflection and refraction of light" | Why transparent things are visible; boundaries |
| High school | Physics: "Properties of waves (refractive index, Snell's law)" | Refractive index; heat haze |
| High school+ | Advanced physics: "Scattering of light" | Why distant mountains look blue |
| University | Electromagnetism and optics (Fresnel equations, dispersion) | What refractive index really is; share reflected |
| Research | Evolution of vision; atmospheric turbulence | Research section |
| ― | Links to daily life | Reflections in windows, haze over distant scenery, summer heat haze |
- Fresnel equations ― Wikipedia (in Japanese)
- Rayleigh scattering ― Wikipedia (in Japanese)
- Hecht, 『光学』 (Optics, Japanese edition; Maruzen) ― a standard textbook on refractive index, reflection and scattering
- National Astronomical Observatory of Japan (国立天文台), ed., 『理科年表』 (Chronological Scientific Tables; Maruzen Publishing) ― refractive indices of air, water and glass
- Bohren, C. F. & Clothiaux, E. E., Fundamentals of Atmospheric Radiation (Wiley-VCH, 2006)
*This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand the mechanism. Refractive index and the degree of scattering vary slightly with the colour of light, temperature and humidity. When handling glass or oil in the activity, take care to avoid breakage and open flames.