Why Glass Does Not Transmit UV Radiation: A Physical and Chemical Explanation

The Role of Glass and Quartz in UV Radiation

When it comes to protecting ourselves from ultraviolet (UV) radiation, one might naturally assume that glass would serve the same protective function as quartz. However, unlike quartz, which allows UV radiation to pass through, typical transparent glass does not permit ultraviolet radiation to pass through it. This article explores the underlying physical and chemical reasons why glass acts as a photoprotective agent to UV radiation.

The Absorption Mechanism of Glass

The reason glass does not transmit UV radiation lies in the molecular structure and composition of the glass. In typical glass, the electrons attached to molecules can absorb radiation at UV wavelengths, but not at visible light wavelengths. This selective absorption is due to the specific energy levels of the electrons in the glass molecules, which align with the energy range of UV radiation but not with visible light.

The Role of Impurities in Glass

The presence of impurities, particularly iron, in glass plays a crucial role in this absorption mechanism. Iron impurities in glass have a broad absorption spectrum that extends into the UV range, making the glass opaque to UV radiation. In the production of windows, for example, iron content is typically around 0.1%, and much of it comes from the cheap mineral sand from mines, not beaches. This iron content is a significant contributor to the glass's ability to block UV radiation.

The Case of Quartz vs. Glass

Quartz, on the other hand, is a natural rock and a semi-precious stone that does allow UV radiation to pass through. When we refer to quartz in the context of glass, we are talking about "fused quartz," which is a form of glass made from natural quartz that has been melted at a very high temperature. Fused quartz has a higher purity level compared to ordinary glass, meaning it contains less iron and other impurities, making it more transparent to UV radiation. However, even fused quartz has limitations in UV transmission compared to fused silica.

Fused Silica and Advanced Glass Technology

The ultimate level of UV transmission is achieved with fused silica, which is produced through a chemical vapor deposition process. This method effectively excludes any metallic impurities, maximizing UV transmission, which is crucial for modern photolithography tools used in semiconductor chip manufacturing.

Additional Insights: Glass and Infrared Radiation

It is important to note that while glass does block UV radiation, it does allow infrared (IR) radiation to pass through. This characteristic is what makes greenhouses work. Greenhouses allow sunlight, which includes UV and visible light, to pass through the glass, while the heat energy (IR radiation) is trapped inside, leading to a warming effect.

In summary, the ability of glass to selectively absorb UV radiation is a result of the specific energy levels of electrons in its molecular structure and the presence of impurities, primarily iron. Understanding these physical and chemical properties is crucial for applications where UV protection is critical.