The Mystery of Light and Carbon:
Why is Graphite Opaque and Diamond Transparent?

Fig (1) Diamond and Graphite and their atomic structures

Light, as we know, is a fascinating wonder that travels as waves in a medium and interacts as photons with matter. Some materials allow it to pass through them with smoothly, while others block it or absorb it. Graphite and diamond for instance are made of the same material carbon but each exhibit an extremely different behaviour when it comes to interacting with light. Graphite seems opaque and black, while diamond is remarkable transparent and shiny. But why does light behave so differently with these two seemingly similar materials? The answer lies in their unique atomic arrangements and bonding properties, let’s find out more about it:
Graphite is commonly found in pencils and other forms. Graphite is a form of carbon where its atoms are joined to three other carbon atoms and this joined-carbon atoms position themselves in a hexagonal arrangement forming a very strong covalent bond between each other. The final arrangement they take is a 2D hexagonal-ring shaped layers, like a stack of sheets of paper or also known as a 2d lattice structure. These layers are lightly held together by the weak Van Der Waals force, which allows them to slip past each other with ease i.e. the pencil trail-marks when drawing or writing with it. This distinctive structure gives graphite its opaque appearance and other characteristics.
So, what happens when light encounters graphite? Well, light interacts with these layers in such a way that causes its photons, which are the particles that make up light, to be absorbed and scattered in various directions. The layered structure of graphite behaves like a barrier that impedes the photons going through the material and as a result what we see is its opaque appearance the absence of light going through. Additionally, a very interesting fact about these loosely held layers, is that this peculiar characteristic allows “graphite to absorb light across a wide range of wavelengths, including visible, ultraviolet, and infrared, contributing even more to its opaque nature” [1].
On the other hand, diamond, another form of carbon with a very distinctive crystal structure, is commonly known for its amazing transparency to visible light unlike graphite. In a diamond crystal structure, each carbon atom forms strong covalent bonds with four neighbouring carbon atoms, and they create a tightly-packed 3D lattice structure and as a result what we get is a highly ordered and dense arrangement of carbon atoms, with no free electrons roaming around or energy states in the band gap, which is the energy range between the valence band, where electrons are bound to atoms, and the conduction band, where electrons move freely. This exceptional structure gives diamonds its exceptional optical properties.
When light passes through diamond, it encounters insignificant obstacles, not layers unlike graphite. The tightly bonded carbon atoms do not absorb light, and the lack of free electrons in the band gap prevents the scattering of photons and as a consequence of this characteristic, the diamond allows light to propagate through it almost freely which results in high transparency to visible light.
In addition to the above-mentioned characteristic, diamond has also another interesting optical property known as total internal reflection which gives it its brilliance sparkles. When light passes from a diamond which has a high index of refraction about 2.3 to a medium with a lower refractive index like air at about 1 and at angle of about 25o , the light reflects back into the diamond instead of going out of it. It goes something like: light! come back in here.
In conclusion, the different ways in which light behaves when it encounters graphite and diamond materials can be explained to their distinct atomic arrangement structures and bonding properties. Graphite’s layered structure and weak van der Waals forces trigger light absorption and scattering which leads to its opaque colour. Conversely, diamond’s tightly bonded, three-dimensional lattice structure with no free electrons lets light to pass through it with very minimal absorption or scattering and thus resulting in its high transparency and unique optical properties.