system
Given the following post, respond as if you're commenting on Reddit:
human
How is it I can see through a Diamond, even though it's the densest form of carbon?

When something is more dense, its atoms are closer together. If that's the case, then why do the closer-together atoms in a diamond allow more light through than the further-apart atoms of coal?
gpt
Atoms are not little opaque balls that block light. Rather, atoms are collections of electric charges (protons and electrons) which can interact with light via their electric charge. The color and transparency of a material is an emergent phenomenon that arises from how the electric field in light interacts with electric charges in the atoms of the material. Whether an object is opaque or transparent is not a fundamental property of its electrons or even its atoms. Rather, it arises from how the electrons are bound in atoms and how the atoms are bound to each other.

Light that encounters a material can either get absorbed, transmitted through, or reflected back. A material is transparent when most of the light that encounters the material is transmitted through. This happens when there is not a strong mechanism in place to absorb or reflect the light. Since light travels forward on its own even when no material is around, transmission is the default behavior for light, i.e. transmission is what always happens when absorption and reflection are poor. Strong absorption and reflection of light by a material usually happen when the electrons in the material couple strongly to the oscillation of the electric field in the light, and are therefore able to redirect the light. The transparency of a material is therefore a function mostly of how strongly or loosely the electrons are bound to one place in the material, and is not a function of how close the atoms are. An extreme example is metals. Metals contain a sea of electrons that are very loosely bound to one place (the conduction electrons). As a result, the electrons are free to respond strongly to the waving electric field in light. Therefore, metals have high reflection and high absorption, and therefore low transmission. In contrast, all the electrons in diamond are very strongly bound, so they cannot interact much with light, so diamond has a high transparency. The whole story is a lot more complex than this because there are so many different ways electrons can form bonds and behave in a material, and because atoms are really quantum objects, but this is a good starting picture.