Why is glass transparent? – Mark Miodownik


Take a look out your window, put on your glasses if you wear them. You might want to grab a pair of binoculars, too, or a magnifying lens. Now, what do you see? Well, whatever it is, it’s not the multiple layers of glass right in front of you. But have you ever wondered how something so solid can be so invisible? To understand that, we have to understand what glass actually is, and where it comes from. It all begins in the Earth’s crust, where the two most common elements are silicon and oxygen. These react together to form silicon dioxide, whose molecules arrange themselves into a regular crystalline form known as quartz. Quartz is commonly found in sand, where it often makes up most of the grains and is the main ingredient in most type of glass. Of course, you probably noticed that glass isn’t made of multiple tiny bits of quartz, and for good reason. For one thing, the edges of the rigidly formed grains and smaller defects within the crystal structure reflect and disperse light that hits them. But when the quartz is heated high enough the extra energy makes the molecules vibrate until they break the bonds holding them together and become a flowing liquid, the same way that ice melts into water. Unlike water, though, liquid silicon dioxide does not reform into a crystal solid when it cools. Instead, as the molecules lose energy, they are less and less able to move into an ordered position, and the result is what is called an amorphous solid. A solid material with the chaotic structure of a liquid, which allows the molecules to freely fill in any gaps. This makes the surface of glass uniform on a microscopic level, allowing light to strike it without being scattered in different directions. But this still doesn’t explain why light is able to pass through glass rather than being absorbed as with most solids. For that, we need to go all the way down to the subatomic level. You may know that an atom consists of a nucleus with electrons orbiting around it, but you may be surprised to know that it’s mostly empty space. In fact, if an atom were the size of a sports stadium, the nucleus would be like a single pea in the center, while the electrons would be like grains of sand in the outer seats. That should leave plenty of space for light to pass through without hitting any of these particles. So the real question is not why is glass transparent, but why aren’t all materials transparent? The answer has to do with the different energy levels that electrons in an atom can have. Think of these as different rows of seats in the stadium stands. An electron is initially assigned to sit in a certain row, but it could jump to a better row, if it only had the energy. As luck would have it, absorbing one of those light photons passing through the atom can provide just the energy the electron needs. But there’s a catch. The energy from the photon has to be the right amount to get an electron to the next row. Otherwise, it will just let the photon pass by, and it just so happens that in glass, the rows are so far apart that a photon of visible light can’t provide enough energy for an electron to jump between them. Photons from ultraviolet light, on the other hand, give just the right amount of energy, and are absorbed, which is why you can’t get a suntan through glass. This amazing property of being both solid and transparent has given glass many uses throughout the centuries. From windows that let in light while keeping out the elements, to lenses that allow us to see both the vast worlds beyond our planet, and the tiny ones right around us. It is hard to imagine modern civilization without glass. And yet for such an important material we rarely think about glass and its impact. It is precisely because the most important and useful quality of glass is being featureless and invisible that we often forget that it’s even there.

100 thoughts on “Why is glass transparent? – Mark Miodownik”

  1. I gave you a thumbs down because you used two models of the atom, both of which are incorrect and impossible. It wouldn't be that much harder to use a correct model to make the same point.

  2. I will discuss this with Santa and the Easter Bunny… just say we don’t know. Making up a story is just wrong.

  3. Glass is transparent due to Freedom of Information laws. Were it not for that, it would be obscured by the alternate reality it would want you to see instead.

  4. Now I understand, though I always believed it by faith, how that the streets of gold in heaven are transparent as glass.
    Revelation 21:21 And the twelve gates were twelve pearls: every several gate was of one pearl: and the street of the city was pure gold, as it were transparent glass.

  5. There are 2 problems with this explanation: 1. We know that speed of light in glass is about 2/3 of this in vacuum . 2. Refraction of light . If, as this video explaining, photons are not interacting with atoms, why we have this 2 effects?

  6. Sort of a beginners' explanation. Your "atom models" are wrong plus no mention of QED (i.e. the real explanation). I guess if I was 6 years old it might be a bit instructive otherwise its heading someone down the wrong path to understanding.

  7. I'm missing something. 2:49 is he saying because the photon of visible light is absorbed, the material is therefore opaque (so what is the net effect of absorbing the photon?) and where the photon of visible light passes though, the material is therefore transparent. so objectively transparent? transparent in total darkness, but in absence of light who can tell, or transparent only in presence of light?

  8. Is glass is not made from a mixture of Sand (SiO2), lime {Ca(OH)2} & Soda {Na2CO3} in difinite proportions?
    Anyhow, glass is complicated type similar to silicates.

  9. Because the atoms in it are vibrating on a frequency so high that if an electromagnet were to reproduce the same frequency everything within that field will become transparent as well.
    But it also bends space and time. The amount of energy required to do this is so high that it would require a nuclear reactor to produce that amount of energy. Luckily the USS Elridge had nuclear reactors onboard.
    I have explained what really happened in the Philadelphia Experiment.

  10. To answer this simple question;
    Because otherwise you wouldn't be able to look through it.
    Defeats the point of a window.
    😉

  11. There's not a High, Intelligent, Designer/Creator of things in and around the universe. It's just random things that just created themselves from NOTHING. Hmmm…
    "Surely, your Lord is Allaah who created the heavens and Earth in six Days and rose over the Throne, disposing the affairs of all things. No intercessor except after His Leave. That is Allaah, your Lord; worship Him (Alone). Then, will you not remember?"

    To Him is the return of all of you. The Promise of Allaah is true. It is He Who begins the creation and then will repeat it, that He may reward with justice those who believed (in God Almighty) and did deeds of righteousness.
    But those who disbelieved will have a drink of boiling fluids and paindful torment because they used to disbelieve."
    (Quraan, Chapter 10)

  12. honestly you should have gone with the energycloud model instead of bohrs simplifyed version makes a nicer visual for the light energy absorbtion part.

  13. Glass is Transparent,because of what it is
    made off.You also have its Structure,
    Properties and many other things as well.
    This applies to all Materials,ie.,Metals,
    Plastics,Composites and also to the
    newer materials.
    Material Science is a foremost Science
    In every aspect of our Living.

  14. Certain kinds of geckos are transparent enough that with the light behind them you can see their internal organs.

    Hey, imagine if humans were like that. 😀

  15. It would be beneficial to add the natural form of glass. When lightening hits sand. As its translucent and solid due to the type of sand

  16. Such a lame way to explain it. I like physics, I’m an engineer, and I still didn’t get it. It would be easier to close my eyes for two minutes and figure it out myself.

  17. My high school told me Silicon Dioxide has no molecule. Unless you wanna call a whole piece of quarz "one huge molecule".

  18. nononono……there is no molecule in Quartz Silicon atoms and oxygen atoms exists with bonds bonding every of them if the quartz is its solid state

  19. If you can't get a sun tan through a window, then why did so many of my classmates come to school with sunburnt skin because they did their homework in front of a closed window?

  20. Wrong. You DO get a sun tan through glass. Ask any car or truck driver who has NO sun protection coated windows in their vehicle about the difference of the tan between their two arms, after driving a while with short arm shirts.

  21. No no no.

    Quartz and typical soda lime or borosilicate glasses have very different uv transmission characteristics. You can most certainly get a sunburn through quartz as it is fairly transparent to parts of the uv spectrum.

    Also… the explanation of why glass is transparent (the part about the atoms of which glass is composed, namely silicon and oxygen) is not correct.

  22. Ted ed team i have a doubt. When the photons fall upon opaque solid all photon energy is absorbed . so why doesnt this opaque solid gets extremely heated. And if the electrons jump, there should be atomic instability in opaque solid???

  23. 2:08 and if you removed all the enpty space from each atom in the empire state building, it would become the size of a grain of rice and still weogh alot

  24. There is clearly a misconception here. There are opaque glasses as well. What is possibly described here is boron-silicate glass, which happened to be transparent in visible spectrum. The explanation about atomic absorption in the glass is also dubious, as color of the glass mostly comes from the impurities, which form vacancies and dislocations (imperfections) inside the glass during manufacturing or ions, which may form right under incoming light. You all seen the Charmeleon glasses, which get dark under sunlight. Here the Cerium atom forms an iron pair with Silver (both are additives to glass composition), which is responsible for absorption of visible light under UV radiation

  25. Sincerely, thank you for making the electrons blurry, people still think things on that scale are solid billiard type balls. I appreciate the less misleading model a ton

  26. Wait you said that you can’t get a suntan through glass. If that’s true then why does my left arm get a tan from sitting on my car door at the base of the window while my right arm doesn’t because it’s not in the sun at all while I’m driving?

  27. Now I want to know who invented glass or discovered how to make it and why they did. How far back in history do we need to go? I know that for a long time Venice in Italy was the only place to get glass that was usable. The glass blowers were all kept on an island so that no one could hire them away by offering them more money. This also helped keep the secrets of how to make glass from spreading around Europe for centuries.
    I definitely want to see the video I described. Thank you
    Cheers

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