Why Is Glass Transparent?
― Made From Sand, Yet It Never Stops the Light
Glass is made almost entirely from sand. Yet sand is not see-through, while glass lets you see clearly to the other side. The answer: light is too weak for glass to bother with, and there are no boundaries inside glass to bounce light back. When both are true, a material is transparent.
It is a rainy day. A child presses their face to the window and gazes outside. Then they turn around and ask: "Why can I see through glass? I can't see through a wall."
You start to say "Because it's glass," and then stop. You remember hearing that glass is made by melting sand. And beach sand is clearly not see-through.
Same material, yet one is clear and the other is not. Follow that difference, and you find the answer to the question: what is transparency?
There are only two reasons for transparency
A material absorbs light only when the light can give an electron inside it a push. Moving the electrons in glass takes a lot of energy, and visible light falls far short. So the light passes through without doing anything.
Every time light reaches the boundary between two materials, it bends or bounces back a little. Glass is uniform throughout, with no tiny boundaries. So light can go straight through to the other side.
Neither reason is enough alone. Even if light is not absorbed, many boundaries make a material cloudy white. And even with no boundaries, if light is absorbed, the material looks colored and dark. Let's take them in turn.
Visible light is "too light" for glass
You can think of light as a stream of tiny packets of energy. The energy of each packet depends on the colour of the light. Packets of red light carry little energy, and packets of violet light carry more.
Inside a material, electrons are bound to atoms. An electron accepts a packet of light only if it receives enough energy to climb a "step" of a fixed height in one go. A packet that cannot reach the step is not accepted. Falling just a little short does not count either.
Look at Figure 1. The height shows the energy of one packet of light. The step that electrons in the main ingredient of glass (silicon dioxide) must climb is said to be about 9. Even the strongest violet in visible light is only about 3. It does not even reach a third of the step, so glass ignores the light. The light has no choice but to pass through.
Slide the control all the way left and you enter the ultraviolet range. Ultraviolet packets carry more energy, so they start to be absorbed by ingredients in window glass. Window glass is said to block much of the shorter-wavelength (higher-energy) ultraviolet that causes sunburn. But it lets a lot of longer-wavelength ultraviolet through, so you can still get sunburnt by a window.
Coloured glass, on the other hand, has small amounts of metals and other additives mixed in on purpose to provide electrons with lower steps. Those electrons absorb only certain colours of light, so the remaining colours are what you see.
Even the same glass turns white as a powder
If electrons were the whole story, sand should be see-through too, since it is the same silicon dioxide as glass. Yet beach sand is not. This is where the second reason, "boundaries", comes in.
Each time light enters glass from air, or leaves glass into air, it bounces back a tiny bit and its direction bends. For a window pane, there are only two boundaries, front and back. Less than a tenth of the light bounces back in total, and the rest goes straight through.
In a pile of sand, or in crushed glass, light hits tiny grains again and again, and is bent and bounced hundreds or thousands of times. Soon the light is scattered in every direction, and some even comes back out the side it came from. Every colour scatters in the same way, so the result looks white (Figure 2).
The same thing happens all around us. Clear ice turns into white shaved ice when it is finely scraped. Clear quartz crystal and whitish sand are made of the same silicon dioxide. Stone and wood are not see-through because they are full of boundaries: tiny crystals, and gaps between cells.
Glass is made by cooling and hardening molten material before crystals can form. So no grain boundaries appear inside, and every part has the same makeup. This "uniform all the way through" structure is what supports its transparency.
Inside a metal are many electrons that are not bound to atoms and can move around. These electrons have almost no "step to climb", so they react at once to light of any colour and bounce it back. That is why metal looks shiny and you cannot see through it. It is the exact opposite of how glass works.
Frosted glass hides what is behind it because the tiny bumps on its surface act as "boundaries" that scatter light. When you wet it, water fills the gaps between the bumps. Water and glass bend light in similar ways, so the boundaries become less noticeable and you can see through much more clearly.
Summary
Glass is transparent because two things are both true: visible light is too weak to move the electrons in glass, so it is not absorbed, and there are no boundaries inside glass to bounce light back. Even with the same material, grinding it to powder adds boundaries and turns it white.
Transparency means light is "ignored",
and also "left undisturbed".
We cover the way boundaries scatter light and make things look white in more detail in Ice Is Clear, So Why Does Snow Look White? For the question of whether glass is a solid or a liquid, see Is Glass a Solid or a Liquid? And for ultraviolet light through windows, Why Can You Get Sunburnt on a Cloudy Day? may also help.
- Put rock sugar and powdered sugar side by side. Both are the same sugar, but the large crystals of rock sugar are see-through and the fine powder is bright white. The finer the grains, the more boundaries there are.
- If you have frosted glass at home, rub a little water onto it with a finger. Only the wet patch lets you faintly see through to the other side. Sticking clear cellophane tape on it works the same way.
- Put one drop of cooking oil on white tissue or paper. The soaked spot turns translucent. The oil fills the gaps (boundaries) between the paper fibres, so the light scatters less.
Do not break or crush glass on purpose to test this. It is dangerous. You can check the same effect with sugar or paper.
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics and chemistry
- High school+Advanced high-school material, or textbook sidebar topics
- UniversityUniversity specialist courses (solid-state physics, optics), not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled
Middle schoolTerms: this phenomenon has names
- Refraction and reflection of light: when light reaches a boundary between materials with different properties, it bends or bounces back. This is a basic property of light taught in middle school.
- Photon: the name for light seen as a "packet of energy". The energy of one packet depends on the colour (wavelength) of the light.
- Amorphous solid: a solid whose atoms set in a jumbled arrangement, rather than forming an orderly crystal. Glass is the classic example, and it has no grain boundaries.
Middle schoolHigh schoolChecking with a formula: how many times smaller is visible light than the step in glass?
We calculate the energy of one packet of light from its wavelength. Then we compare it with the step that electrons in glass must climb (about 9 electron-volts). The electron-volt is the energy one electron gains from a voltage of 1 volt, and it is a "ruler" for the atomic world.
| In symbols | E ＝ h × c ÷ λ |
| In words | Energy of one packet of light = Planck constant × speed of light ÷ wavelength of light |
| Where it comes from | The energy of a packet of light is proportional to its frequency (Einstein's light-quantum hypothesis). Frequency is "speed of light ÷ wavelength", so the shorter the wavelength, the greater the energy. |
| E | Energy of one packet of light (in electron-volts) |
| h | Planck constant (a fixed number of nature) |
| c | Speed of light (about 300,000 km per second) |
| λ | Wavelength of light (in nanometres; 1 nanometre is one millionth of a millimetre) |
| h times c (in electron-volts and nanometres) | about 1240 |
| Wavelength of green light | 550 nanometres |
| Shortest visible wavelength (violet) | about 400 nanometres |
| Step for electrons in quartz glass | said to be about 9 electron-volts |
| Rough point where window glass starts absorbing | said to be about 4 electron-volts |
| Energy of one packet of green light | 1240 ÷ 550 ≒ 2.25 electron-volts |
| Energy of one packet of violet light | 1240 ÷ 400 ＝ 3.1 electron-volts |
| How many times the strongest violet is the step | 9 ÷ 3.1 ≒ 2.9 times |
| Wavelength where window glass starts absorbing (formula used in reverse) | 1240 ÷ 4 ＝ 310 nanometres |
Even the strongest violet in visible light reaches only about a third of the step. Window glass starts absorbing at around 310 nanometres, which lies in the invisible ultraviolet range. So all visible light passes through, and glass looks transparent.
High schoolHigh school+Surface reflection and the refractive index
High schoolThe number that shows how readily a material bends light is the refractive index. It is about 1 for air and about 1.5 for window glass. The bigger the difference in refractive index across a boundary, the stronger the reflection. For light hitting head-on, one glass surface is said to reflect about 4%, which adds up to under a tenth for the front and back together.
High school+Frosted glass turns clear when you wet it with water (refractive index about 1.33) because the difference in refractive index across the boundary shrinks from about "0.5" to about "0.17". The idea of erasing the difference at a boundary to make something clear is called refractive index matching. It is also used in techniques that make living tissue transparent for viewing under a microscope.
UniversityBand theory, and the limit of optical fibres
In university solid-state physics, the range of energies electrons can have is called an energy band, and the "step" where electrons cannot exist is called the band gap (forbidden band). If the energy of a photon is smaller than the band gap, it cannot lift an electron to the upper band, so the light is not absorbed. This is the basic mechanism of a transparent insulator. Metals have no band gap, and their free electrons reflect light.
Still, no material is perfectly uniform. As glass sets, it traps very slight variations in density. The scattering of light by these variations is called Rayleigh scattering. In optical fibres for communication, Rayleigh scattering is the main source of loss, and even at the wavelength with the least loss, the light is said to weaken by about 0.2 decibels (about 5%) per kilometre.
📖 Derivations and further reading: Band gap (Wikipedia, in Japanese) ／ Photon (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- Why can glass harden without crystallising? The "glass transition", in which a cooling liquid suddenly slows down, is said to remain one of the big open problems in physics.
- How transparent can an optical fibre become? Researchers are studying ways past the Rayleigh scattering barrier, such as materials with less density variation and fibres with a hollow core.
- Technology to make opaque materials transparent. "Transparent wood", made by removing the compounds that give wood its colour and filling the gaps with resin, and transparent ceramics with no grain boundaries have been developed. How far they can be made practical is still under research.
So the content of this article is also "an explanation of what is known so far". The basics of transparency are understood, but ways to push it to the limit are still being sought.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, year 1: "Reflection and refraction of light" | Light bends and bounces at boundaries (Figure 2) |
| High school | Physics: "Refractive index of light", "Photon energy" | Checking with a formula, surface reflection |
| High school+ | Advanced physics and chemistry: "Amorphous solids", "Refractive index matching" | Frosted glass and water, how glass is made |
| University | Solid-state physics: "Band theory"; optics: "Rayleigh scattering" | The step (band gap), optical fibres |
| Research | Glass transition, new transparent materials | The research section |
| ― | Links to daily life | Sunburn by a window, frosted glass, coloured glass |
- Wikipedia, "Band gap" (バンドギャップ), in Japanese
- Wikipedia, "Photon" (光子), in Japanese
- Sakka Sumio (作花済夫), Fundamentals and Applications of Glass Science (ガラス科学の基礎と応用), Uchida Rokakuho (内田老鶴圃)
- High-school physics textbooks (units on refraction of light and photon energy)
- C. Kittel, Introduction to Solid State Physics, Wiley (the standard textbook on band theory)
*This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. Values for absorption and reflection in glass vary with the type and thickness of the glass. Do not try experiments that involve breaking or crushing glass, as there is a risk of injury.