Science Facts

Why Is Glass Transparent? – Structure & Causes

Glass Transparency

Glass has a strange structure since it’s made by melting and cooling silicon dioxide. Silicon dioxide is formed when silicon and oxygen react together in the Earth’s crust. As they react, the molecules arrange themselves in a regular crystal known as quartz. However, when it is melted, it loses this crystal structure as it cools and the molecules lose energy. They’re not able to move back into the orbit of the crystal. So instead, form a random structure like that of a liquid. This liquid light structure is called an amorphous solid. It gives the glass a uniform surface meaning and does not scatter the lime in different directions.

In an atom, the electrons around the nucleus are at specific energies called energy levels. The lowest energy shelf is called the ground state. Electrons can move up to a higher shelf if they have enough energy. Otherwise, their energy is too small, and they can’t quite reach the shelf. If photons of light have the same energy as the difference in energy between the two energy shells, then the electron can absorb the photon and gain energy. It gives the electron a leg up onto the higher shelf. Unfortunately no! If the photon’s energy is not the same as this difference, it will just pass straight through, and the electron can’t absorb it and so can’t move up. This is what happens in the glass.

What is glass?

Glass is primarily made of silica dioxide, which can be a white powdery substance. It is also the main constituent of sand. This can be added to things like sodium carbonate, lime, oxide, and even iron. However, most things are added either to lower the melting point or change other properties within the glass itself.

Glass is technically any rigid amorphous solid, meaning its atoms and molecules aren’t arranged in an orderly structure. But rather, in whatever random arrangement they happened to be in when the material cooled and solidified. It’s as though a liquid just stopped moving all of a sudden. Unlike ice, the water molecules tug on each other and lock themselves into a repeating crystal pattern. As the glass cools, its molecules contract until they stop moving altogether.

How glass is made?

Human bodies aren’t transparent to visible light. Visible light and x-rays are different forms of electromagnetic radiation, with different wavelengths and energies. So what’s the difference? Glass is transparent to visible light. It is made up of a bunch of silicon and oxygen atoms. Same as this stuff, sand!

By melting down sands into a liquid and cooling them down fast, they froze in place, in a sort of an organized jumble. Wave, quantum-y electron clouds surround all those atoms. But the electrons around a nucleus can’t be just anywhere. They live on specific energy levels. When a photon comes by, with exactly the right amount of energy, it gets absorbed, bumping an electron to a higher energy level. But if that photon doesn’t have just the right amount of energy, it passes right by. For the particular atoms that make up glass.

Structure of glass

Inside the earth’s crust, silicon and oxygen react together to form silicon dioxide, and the molecules of this mixture come together to create a common crystalline form called quartz. It is the main ingredient of most types of glass. When we burn the quartz at a high temperature, the heat energy puts pressure on the molecular chain of silicon dioxide. It breaks them apart and converts them into flowing liquid. When this liquid silicon dioxide cools down again, it does not transform into a solid crystal again.

Structure of glass
Structure of glass

That’s because as the molecules lose energy, they are unable to move into an ordered position and form a chain as they used to be before. And it converts into something called amorphous solid. A material with a disordered composition of a liquid allows the molecules to fill any gaps. This makes the surface of the glass uniform, allowing light to pass through it without getting scattered in different directions.

Why is glass transparent?

The energy levels are so far apart that none of the visible light photons have enough energy to move the electrons up. So they all pass through unabsorbed. Shortly, It is the reason for transparency.

  • Electrons do not have size. They can only occupy specific energy levels. If energy is definite, the position is not.

If electrons are simultaneously everywhere, then the photon will hit one no matter what, and electrons absorb the photon. They can only absorb certain types of light. Jumping up energy levels is not the only way electrons can absorb light. If it were, there would be no such thing as reflection or refraction.

Energy gap
Energy gap

There are so many photons hitting so many electrons that photons are just going everywhere inside the glass. Those photons interfere and cancel each other in most places where they don’t cancel the light. Traveling in a new direction is determined by how the atoms are organized. When atoms bond to form solids, the energy levels move closer together to form bands. This leaves large energy gaps between them.

  • The gap between bands in glass is about 9 electron volts which are too big for visible light. To excite electrons, the photon has to have an energy of at least 9 electron volts. It means its wavelength must be at most 138 nanometers. That’s ultraviolet light.

If visible light can’t excite electrons, then it won’t get absorbed as thermal energy. It still gains absorbed just as the vibration and the electrons. Since the electrons always reject those photons, they make it out the other side transparency.

The energy levels are so far apart that visible light doesn’t have enough energy to boost the electrons up to the next level. That’s why visible light passes right through! But photons of UV light do have the right amount of energy to power up those electrons. And they get absorbed. This is why glass is opaque to most UV! And why it’s hard to get a sunburn through a window.

Visible light has a higher frequency, and so it must have higher energy photons than visible light. This is a result of the black Einstein equation (E = hf). So maybe one of them has not too much energy but not too little. Their energy is just right when it turns out for glance the Goldilocks region of energies lies in the ultraviolet. It means the ultraviolet photons are absorbed by the electrons and so don’t pass through.

How transparent something is depends on this relationship between light energy and an atom’s electrons. Different elements have different energy requirements for their electrons to absorb light. Like how when visible light hits atoms, it’s absorbed. Some light might get through a few top layers of skin cells, but within a few millimeters, all the photons get gobbled up. That’s why everything is not transparent.

Frequently asked questions

How does the light pass through the glass?

To find the answer, we need to dig deeper into the glass to observe the atoms of silicon dioxide. Atoms are made of a nucleus in the center surrounded by electrons orbiting around them. But an atom is mostly empty, and there is plenty of gap between the orbits of electrons. This empty space leaves a lot of room for the particles of light called photons. It passes through the atomic structure of glass without getting absorbed by electrons, and the glass looks transparent.

Why do other objects look opaque and not transparent?

That’s because the gaps between the electrons of an opaque object are tiny. These tiny gaps do not allow the photons to pass through it and absorb electrons, making the object look opaque. High-temperature glass can naturally form when sand is struck by lightning. Also, another amazing fact about glasses that when it breaks the cracks can move at the speed of 3,000 miles per hour which is five times faster than the average airplane which travels at 575 miles per hour.

Does light slow down in glass?

Dividing the distance by time gives us a speed less than the speed of light. It gives you an average speed. When the light beam comes into contact with the glass, its atoms start to vibrate in response. That vibration causes all those atoms to emit their light. Most of the light cancels except for one particular direction: the forward direction. When combining all those waves, they look like one beam traveling slower than the speed of light. But is the light traveling slower?

That depends on what you mean by “light.” It’s a language issue. One single photon is “light,” but this beam has a bunch of photons in it. If you also refer to that beam as “light,” then, sure, “light” slows down in materials. But, It’s physically impossible for light not to go at the speed of light. Beams of light don’t always have the same properties as their photons.

Is glass a solid?

Glass isn’t technically a solid or liquid. It’s an amorphous solid, meaning it lacks the crystalline or polycrystalline repeated structures of other solids. This lack of a pattern and weak chemical bonds explains how glass can behave like a brittle solid. Also, it accounts for the glass-liquid transition, which is when heated glass is between a supercooled liquid state and a solid state. It behaves like a rubbery viscous liquid.

The molecules on the surface of glass never actually stop moving, though, even when cooled. They do move extremely slowly, but they move nonetheless. A common myth about glass is this movement. Antique glass has sagged over the years. And that’s why many old buildings and church windows have glass that’s thicker on the bottom.

That’s likely just an imperfection in how the glass was made because for a flat pane of glass to sag at room temperature would take longer than the universe’s age. Plus, even older glass from Egypt doesn’t have that sagged quality, so that myth goes. The lack of a crystalline structure is often used to explain glass transparency.

Why does light change direction in glass?

Light traveling through the air will change its direction when it hits the surface. The amount that its direction will change depends on the angle at which it hits the surface. It changes the incident angle, and the transmitted angle will change correspondingly. Those angles are related through a physics formula called Snell’s Law, named after Dutch astronomer Willebrord Snellius, who lived in the 1600s.

Three commonly suggested explanations are wrong or at least incomplete. They are Fermat’s Principle, the analogy of soldiers marching, and Huygens’s Principle. Fermat’s principle says that light will travel from one point to another, taking the minimum amount of time. Light follows the path of the shortest time.

After Dutch physicist Christian Huygens, Huygens’s principle depends crucially on the wave nature of light. This principle is called diffraction, and it’s universal to all waves. If waves hit more than one gap, each gap will allow waves to pass through and make ringlets. The waves gap can interfere with one another, with the waves either adding to or subtracting from one another.

When the peak of two waves encounters one another, the result is a single and bigger wave. And, when a peak of one wave hits the trough of another wave, they can cancel each other out entirely. What Huygens claimed is that there is no need for gaps. Each spot on a wave was acting like it went through a gap and spread out.

Light enters glass or water or any transparent medium, and it bends. The reason that it bends is that the epsilon in glass is bigger than in air. It’s there because of how the electric fields from the light interact with matter in the glass. The glass has charged in it, but they’re arranged randomly. The electric field makes the charges move around, which sets up a counterbalancing electric field from the charges.

The result is that the electric field in the glass is lower than in the air. It is because of how the electric field from the glass is in the opposite direction. And this is the reason that the perpendicular electric field is lower in the glass.


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Sources:

Zallen, R, The Physics of Amorphous Solids. New York: John Wiley. pp. 1–32. ISBN 978-0-471-01968-8.
Cusack, N.E., The physics of structurally disordered matter: an introduction. Adam Hilger in association with the University of Sussex press. p. 13. ISBN 978-0-85274-829-9.
Scholze, Horst, Glass – Nature, Structure, and Properties. Springer. pp. 3–5. ISBN 978-0-387-97396-8.
Elliot, S.R., Physics of Amorphous Materials. Longman group ltd. pp. 1–52. ISBN 0-582-44636-8.
Neumann, Florin. “Glass: Liquid or Solid – Science vs. an Urban Legend.”
Gibbs, Philip. “Is glass liquid or solid?”.

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