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How can amorphous solids be transparent or opaque?

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Amorphous solids, which lack the long-range atomic order found in crystals, exhibit a wide range of optical properties, including both transparency and opacity.123

Whether an amorphous solid is transparent or opaque depends on how the material interacts with light across the electromagnetic spectrum, as described by its optical spectra. Transparency occurs when a material allows light to pass through without significant absorption or scattering. For example, oxide glasses like silicate glass are engineered for high transparency, making them ideal for windows and fibre-optic waveguides.2

Conversely, an amorphous solid may appear opaque if its chemical composition or structural features cause it to absorb or scatter visible light. Just as with crystalline materials, amorphous solids can be insulators, semiconductors, or metals, and these internal electronic structures dictate their ability to transmit light. While the absence of long-range order eliminates the sharp peaks seen in the optical spectra of crystals, the fundamental physical principles governing light absorption and reflection remain applicable to amorphous substances.24