What can go through glass without breaking it?

The ability to see through a pane of glass is a common marvel, and it all comes down to the fascinating interaction between light and the material's atomic structure. Light, composed of tiny energy packets called photons, can interact with the electrons orbiting the atoms within a substance. For a material to absorb light, the incoming photons must possess precisely the right amount of energy to excite an electron, causing it to jump to a higher energy level. In glass, the electrons are tightly bound, and there's a significant energy gap between their normal state and the next available energy level. Crucially, the photons that make up visible light simply do not carry enough energy to bridge this gap and excite the electrons in glass.

Because visible light photons lack the necessary energy to be absorbed, they are not captured by the electrons in the glass. Instead, they pass right through the material with minimal interference. This is what makes glass transparent. In contrast, opaque materials, like wood or metal, have electrons that readily absorb or reflect visible light photons, preventing light from passing through. Translucent materials, such as frosted glass, allow some light to pass, but the light is scattered, resulting in a blurred image.

This unique property of transparency in glass is also partly due to its nature as an amorphous solid. Unlike crystalline solids with highly ordered atomic structures that might scatter light, the randomly arranged atoms in glass allow light to transmit smoothly. While glass is transparent to visible light, it's important to note it's not transparent to all forms of electromagnetic radiation; for instance, it absorbs much of the sun's ultraviolet light, protecting us from those higher-energy rays. This makes the clear view through a window an everyday testament to the specific energy interactions between light and matter.