The statement that most glasses are mainly made of SiO2 (silicon dioxide) and Na2O (sodium oxide) is generally accurate. However, the information about the band-gap and absorption of wavelengths lower than 310 nm requires further clarification.
SiO2 is the primary component of many glasses, such as soda-lime glass, borosilicate glass, and quartz glass. Sodium oxide (Na2O) is often added as a flux to lower the melting point of the glass during manufacturing. Other oxides, such as calcium oxide (CaO) and aluminum oxide (Al2O3), may also be present depending on the specific glass composition.
Regarding the band-gap and absorption, it's important to note that the band-gap of SiO2 itself is relatively large, in the range of 8-9 eV, which corresponds to the absorption of ultraviolet (UV) light with shorter wavelengths than 150 nm. This makes pure SiO2 transparent to visible light. However, the addition of other oxides, such as Na2O, can modify the optical properties of the glass.
The presence of certain impurities or dopants in glass can indeed alter its band-gap and optical properties, allowing absorption of shorter wavelengths. For example, certain transition metal ions or rare earth ions can introduce energy levels within the band-gap, resulting in absorption of visible or ultraviolet light.
It's worth noting that glasses used for specific applications, such as optical fibers or UV-blocking glasses, can be engineered to have specific absorption properties within desired wavelength ranges. Therefore, while it is possible to tailor glass compositions to absorb specific wavelengths, the general statement that SiO2 and Na2O glasses have a band-gap in the 4 eV range and can absorb wavelengths lower than 310 nm would require additional information or context to determine its accuracy in a specific case.