How does light interact with matter?
To answer this question, we first need to understand some concepts about light and matter.
For the purpose of this question, we may think of light as made up of little packets of energy called photons. These photons have more energy the higher the its frequency. As an aside, the frequency of photons also determines its colour.
In any matter, we will find electrons surrounding the nucleus of its atoms. An electron may move to a different energy level, or be 'excited', if it is exposed to a specific amount of energy from a photon. The amount of energy required to excite an electron depends on the structure of its atoms, and is called the 'energy gap'.
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| When a photon has the exact energy required to excite' an electron, it pushes the electron to a higher orbit. |
When a photon 'touches' a solid, it may be absorbed, reflected or simply pass through.
The photon is absorbed
This happens when the photon has the exact amount of energy required to excite the electron. In most solids, this energy is dissipated as vibrations in the atoms.
The photon is reflected or the photon passes through
This happens when the photon does not have the exact amount of energy required to excite an electron in the solid. If this is the case, the photon might 'bounce off' one of the electrons, or simply pass through the solid.
The energy gap of the electrons in glass is higher than the energy of visible light, and so visible light passes through the glass making it transparent.
Of course this leaves us more questions than answers. Why do photons pass through, and not reflect on, glass?
What is glass?
Glass is actually quite a special kind of solid. Most solids that do not absorb visible light have crystalline structures. The internal edges of this structure causes unabsorbed light that hits them to be reflected.
On the other hand, glass is a solid that has a crystalline structure which does not have the same internal edges that reflect light. Glass gets this structure when it is melted and rapidly cooled, allowing its internal molecules to arrange themselves randomly like a liquid, but with the rigidity of a solid.
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| The crystalline structure of a quartz reflects light unlike that of the structure in glass |
So there you have it. Glass is transparent because the energy gap in its electrons is high enough, and it has a structure that allows light to pass through unhindered.
