Imagine if the Earth were a giant atom. Its nucleus would be so small it could fit inside a baseball stadium! Everything outside the stadium, like the rest of the planet, is where electrons hang out in a wave-like, quantum cloud. This might make you wonder: if atoms are mostly empty space, why can’t light just pass through things like bricks, steel, or even chocolate ice cream? Why aren’t we see-through?
Let’s dive into this mystery. Our bodies aren’t transparent to visible light, but they are to X-rays. Both visible light and X-rays are types of electromagnetic radiation, but they have different wavelengths and energies. So, what’s the difference?
Take glass, for example. It’s transparent to visible light. At the atomic level, glass is made of silicon and oxygen atoms, similar to sand. When sand is melted and cooled quickly, its molecules form a disordered but organized structure. These atoms are surrounded by quantum electron clouds, and electrons exist at specific energy levels around the nucleus.
When a photon (a particle of light) comes along with the right amount of energy, it can be absorbed by an electron, causing the electron to jump to a higher energy level. If the photon doesn’t have the right energy, it just passes by. Think of it like this: if I’m an electron, I need just the right energy to jump to a higher level. Too little or too much energy, and I stay put.
In glass, the energy levels are spaced so that visible light doesn’t have enough energy to move the electrons to a higher level. That’s why visible light passes through glass. However, UV light has the right energy to excite those electrons, making glass opaque to most UV light. That’s why you can’t get a sunburn through a window.
Whether something is transparent depends on how light energy interacts with an atom’s electrons. Different elements have different energy needs for their electrons to absorb light. For example, when visible light hits our skin, it’s absorbed. Some light might get through a few layers of skin cells, but most photons are absorbed within a few millimeters. That’s why we’re not transparent. But with higher energy waves like X-rays, we are transparent.
Now, if atoms are mostly empty, why don’t we just pass through everything? Why can we sit on chairs or kick balls? When you touch something, your finger and the object are made of atoms surrounded by negatively charged electrons. As they get close, the electron clouds repel each other due to electrostatic repulsion. This repulsion is what you feel as touch.
Touching doesn’t mean getting atoms to zero distance; it means getting them as close as physics allows. Electrons can also interact in another way. Two negatively charged electrons can share the same energy level if they have opposite spins. Sometimes, electrons in different atoms overlap their wave-like properties, forming covalent chemical bonds. These bonds are in your body, in the molecules that keep you alive, and in materials like glass that let light pass through.
I hope this explanation makes the science a bit clearer. Remember, the world of atoms and light is full of fascinating mysteries. Keep exploring and stay curious!
Using craft materials like styrofoam balls and pipe cleaners, create a model of an atom. Label the nucleus and electron cloud. This will help you visualize how much empty space there is in an atom and why we aren’t transparent.
Gather different materials such as glass, plastic, and paper. Shine a flashlight through each material and observe which ones allow light to pass through. Record your observations and discuss why some materials are transparent while others are not.
Play a game where you act as photons and electrons. Use colored cards to represent different energy levels. Try to match the right photon energy with the electron’s energy level to “jump” to a higher level. This will help you understand how light interacts with atoms.
Research how X-rays work and why they can pass through the human body. Create a poster explaining the difference between X-rays and visible light, and how this relates to transparency.
Use balloons to demonstrate electrostatic repulsion. Rub two balloons on your hair and try to bring them close together. Observe how they repel each other, similar to how electron clouds repel, explaining why we can touch objects without passing through them.
Thank you to CuriosityStream for supporting PBS. If the Earth were one giant atom, its nucleus could fit inside a baseball stadium! Everything outside the stadium—the rest of the planet? That’s where the electrons live, in a sort of wave-like, quantum cloud. The stuff that makes up matter doesn’t contain much actual substance. But if an atom is just a tiny nucleus surrounded by a wave-like, quantum cloud of mostly nothing, it makes you wonder: Why doesn’t light pass right through the atoms in bricks, steel, or chocolate ice cream? Why aren’t we transparent?
Hey smart people, Joe here. So, why aren’t we transparent? Well, we are—if you’re an X-ray! Our bodies just aren’t transparent to visible light. Of course, visible light and X-rays are both different forms of electromagnetic radiation, with different wavelengths and energies. So what’s the difference?
Let’s break it down. Glass is transparent to visible light. If we zoom down to the atomic level, we see that glass is made up of silicon and oxygen atoms—just like sand! When that sand is melted down into a liquid, those molecules leave their perfectly repeating crystal shape and become disordered. When we cool them down quickly, they freeze in place in a sort of organized jumble. All those atoms are surrounded by wave-like, quantum electron clouds. But the electrons around a nucleus can’t be just anywhere; they exist on specific energy levels—think of them as different distances from that tiny nucleus.
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. Imagine I’m an electron. I’m hanging out at a low energy level, wanting to move to a higher one. To make it happen, I need just the right amount of energy in my jump. Too little, and I don’t make it. Too much, and… oops. Just right.
For the particular atoms that make up glass, the energy levels are so far apart that visible light doesn’t have enough energy to boost those electrons up to the next level. That’s why visible light passes right through! However, photons of UV light do have the right amount of energy to excite those electrons, which is why glass is opaque to most UV light. That’s also why it’s hard to get a sunburn through a window.
How transparent something is depends on the relationship between light energy and an atom’s electrons. Different elements have different energy requirements for their electrons to absorb light. For example, when visible light hits my 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 absorbed. That’s why I’m not transparent. But hit me with higher energy waves, like X-rays, and I am transparent.
Now, thinking about how atoms are mostly empty clouds makes me wonder something else: Why am I even here? Why aren’t the mostly empty atoms in my feet passing right through the mostly empty atoms in the ground? Why can I sit on a chair, kick a ball, or interact with objects?
Let’s say I want to touch something. My finger and the object are both made of countless atoms, and all those atoms are surrounded by negatively charged electrons. As the two objects get close enough, the negatively charged electron clouds at both surfaces repel each other, thanks to electrostatic repulsion. The sensation of touch is caused by this repulsion acting on pressure-sensitive nerves in my skin.
Touching something doesn’t mean decreasing the distance between me and the object to zero; it just means getting my atoms and the other object’s atoms as close as physics will allow. There’s also another way that electrons can interact. It is possible for two negatively charged electrons to occupy the same energy level, as long as they have opposite spins. Sometimes, electrons in two different atoms can be close enough that their wave-like properties overlap. That’s how covalent chemical bonds exist, which is pretty convenient.
These bonds are found in your body, in the molecules that keep you alive, and in the materials that allow light to pass through, like the glass of the screen between us right now. I hope this explanation makes the science a little clearer. Stay curious!
Atom – The smallest unit of a chemical element that retains its chemical properties. – Everything around us is made up of atoms, which are the building blocks of matter.
Electron – A subatomic particle with a negative charge that orbits the nucleus of an atom. – Electrons move around the nucleus of an atom and are involved in forming chemical bonds.
Light – A form of energy that travels in waves and can be seen by the human eye. – When light passes through a prism, it separates into different colors.
Energy – The ability to do work or cause change, often measured in joules or calories. – In physics, energy can be transformed from one form to another, such as from potential energy to kinetic energy.
Transparent – Allowing light to pass through so that objects behind can be distinctly seen. – Clear glass is transparent, which is why we can see through windows.
Glass – A hard, brittle substance typically made from sand, used in windows and bottles. – Glass is often used in laboratories because it is transparent and can withstand high temperatures.
Wavelength – The distance between successive crests of a wave, especially in sound or light. – Different colors of light have different wavelengths, which is why they appear as separate colors in a rainbow.
Photon – A particle representing a quantum of light or other electromagnetic radiation. – Photons are responsible for carrying energy from the sun to the Earth.
Opaque – Not allowing light to pass through; not transparent or translucent. – A wooden door is opaque, so you cannot see through it.
Chemistry – The branch of science that studies the properties, composition, and behavior of matter. – In chemistry class, we learned how different substances react with each other to form new compounds.
