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Thermal shock refers to the stress experienced by a material when it is subjected to a rapid change in temperature. This stress can cause materials to expand or contract at different rates, leading to cracks or fractures.

Borosilicate glasses are a type of specialty glass that has boron trioxide (B_2O_3) and silica (SiO_2) as its main constituents. Boron trioxide acts as a fluxing agent and helps to reduce the melting point of silica, making the glass easier to form. Borosilicate glasses have several important properties, such as a low coefficient of thermal expansion (CTE), high chemical resistance, and high durability.

Fused silica is a high-purity glass composed almost entirely of silica (i.e., silicon dioxide, SiO_2). It is produced by fusing high-purity silica sand or other silica sources at high temperatures. Fused silica has a very low coefficient of thermal expansion (CTE), high chemical resistance, and excellent optical properties.

Both borosilicate glasses and fused silica have a low coefficient of thermal expansion. This means that they don't expand or contract significantly when their temperature changes, making them less likely to crack or fracture under thermal shock. In contrast, regular glasses tend to have higher coefficients of thermal expansion, making them more susceptible to cracking under sudden temperature changes.

Borosilicate glasses and fused silica also have high chemical resistance. This means that they don't react easily with other substances and materials, making them more stable and less likely to weaken due to chemical interactions. Additionally, both materials are highly durable and resistant to cracking, which further enhances their resistance to thermal shock. In conclusion, borosilicate glasses and fused silica are resistant to thermal shock due to their low coefficient of thermal expansion, high chemical resistance, and high durability. These properties make them less prone to cracking or fracturing when subjected to sudden temperature changes, thus allowing them to withstand thermal shock better compared to regular glasses.