Why is glass transparent? Glass is transparent because light photons don’t have enough energy to excite the molecules in the glass.
There are two things we need to look at here. What is glass and what is light?
Glass is a non-crystalline amorphous solid. There’s a bit to unpack there. Non-crystalline refers to the structure of the molecules inside the glass. On a basic level, glass starts out as silica, which has a crystalline structure because all of its atoms are aligned. Silica is the quartz crystals that you can see in sand. When the silica is heated to over 1700˚C, it melts and the structure changes. The molecules no longer have a rigid structure and the molecules are randomly arranged. This is what makes glass an amorphous solid. It has the properties of a solid, but the structure of a supercooled liquid.
Light is the part of the electromagnetic spectrum that we can see. Energy travels across the universe from different sources. In our solar system, it comes from the sun. The energy travels in waves with wavelengths that reflect the amount of energy that the radiation has. Radio waves are low energy and have long wavelengths. Gamma rays are very high energy and have an extremely short wavelength. The light that is visible to us is the tiny bit in the middle.
There are three ways that substances can react to light. The first way is to absorb the light. The light photon hits the atom and gives up its energy to the electrons around the nucleus. This creates energy. Plants do this when they photosynthesize. If an object absorbs all of the visible light, we see it as black.
The second way is to reflect the light. The photon hits the atoms, the energy is absorbed, but a photon of equal energy is fired back. Anything you can see does this. A door. Your hand.
The third way is to let the light pass through. The photon doesn’t interact with any electrons and passes straight through. This is what glass and many liquids do. But why?
All of the molecules in materials have energy bands. There is a certain amount of energy required to make an electron in the molecule move from a lower energy band to a higher one. When light photons enter an object that is not transparent, the energy in the photon is enough to move an electron to a higher band and the energy from the light is absorbed, meaning the object is not transparent. Because the molecules in glass are arranged in a different way, the energy required to move an electron to a higher energy plane is much higher. Visible light doesn’t have enough energy to make the electron change bands, so it is not absorbed, and it passes through the glass.
The electrons in the glass can be made to jump to a higher band if there is enough energy. Ultraviolet light has a smaller wavelength than visible light. That means it has more energy and it has enough energy to make the electrons in light move to the higher energy band. Thus, ultraviolet light is absorbed by glass and cannot pass through it.
So, on a related note, why is a window not transparent when it is dark outside. Well, the answer is that it is, really, it is just harder for us to see. Throughout the day, about 95% of the light photons pass through the window and about 5% are reflected back through chance encounters with molecules. You have to remember that light goes both ways through a window. The light from inside goes out and the light from outside comes in. By day, we can’t see this 5% of reflected light because the light coming through the window from the other side is much brighter. However, at night, there is no light coming through the window from the outside, so the 5% that is reflected becomes very visible. There is no change, it is just easier for us to see. If you switch the lights off in the room, the window becomes transparent again.
So, visible light doesn’t have enough energy to move the electrons in glass to a higher energy plane and it simply passes through, which is why glass is transparent. And that is what I learned today.
Sources
https://www.azom.com/article.aspx?ArticleID=8141
https://www.scientificamerican.com/article/what-determines-whether-a/
https://science.howstuffworks.com/question404.htm
https://physics.stackexchange.com/questions/7437/why-is-glass-transparent
https://www.corning.com/worldwide/en/innovation/materials-science/glass/how-glass-made.html
https://www.explainthatstuff.com/glass.html
https://andor.oxinst.com/learning/view/article/what-is-light
https://andor.oxinst.com/learning/view/article/absorption-of-light