Why Glass is Transparent?
Why Glass is Transparent?
Glass is a solid. It is made up of molecules which is similar to what many other solids are made of. Then why is it transparent? Is it that the color of the molecules in glass are transparent and those in other solids are not?
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 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 ingredients in most type of glass. Of course, you probably noticed that glass isn't made of tiny bit 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 bond 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 it called and amorphous solid. This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly. As a result, glasses are mechanically rigid like solids, yet have the disordered arrangement of molecules like liquids. 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 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.
To understand that, we have to understand what glass actually is, and where it comes from. It all begins in the earth 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 ingredients in most type of glass. Of course, you probably noticed that glass isn't made of tiny bit 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 bond 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 it called and amorphous solid. This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly. As a result, glasses are mechanically rigid like solids, yet have the disordered arrangement of molecules like liquids. 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 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 row 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, observing 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 happen 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. Transparency of any material depends on the amount of light the material allows to pass through it, for glass or glass-like materials has high transparency that makes them less reflective. If the material is absorbing light then also it can not be considered transparent because it is not letting the light to transmit. So for glass, there are less absorption and reflection and high transmittance. These properties make a glass transparent material.
The answer has to do with the different energy levels that electrons in an atom can have. Think of these as different row 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, observing 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 happen 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. Transparency of any material depends on the amount of light the material allows to pass through it, for glass or glass-like materials has high transparency that makes them less reflective. If the material is absorbing light then also it can not be considered transparent because it is not letting the light to transmit. So for glass, there are less absorption and reflection and high transmittance. These properties make a glass transparent material.
Let's consider a photon moving toward and
interacting with a solid substance.
One of three things can happen:
- The substance absorbs the photon. When the energy level of the electron is similar to the energy of the light, the electron absorbs the light. Armed with this extra energy, the electron is able to move to a higher energy level, while the photon disappears.
- The substance reflects the photon. To do this, the photon gives up its energy to the material, but a photon of identical energy is emitted.
- The substance allows the photon to pass through unchanged. Known as transmission, this happens because the photon doesn't interact with any electron and continues its journey until it interacts with another object.
Glass, of course, falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a glass electron to a higher energy level.
Glass surrounds us everywhere we go. It has become so important in our daily lives. Without glasses, we wouldn't have telescopes or microscopes. Biology, astronomy, and navigation would all be back in the dark ages, as they all rely heavily on magnifying devices. With biology so limited, healthcare would also be severely limited. And photography would largely have been limited to pinhole cameras. 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.
Glass is a solid? It is
made up of molecules which is similar to what many other solids are
made of? Then why is it transparent? Is it that the color of the
molecules in glass are transparent and those in other solids are not?
Reference https://www.physicsforums.com/threads/why-is-glass-transparent-and-solid.166973/
Reference https://www.physicsforums.com/threads/why-is-glass-transparent-and-solid.166973/
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