Glass is a poor conductor of electricity and is thus used as an electrical insulator. It is also a poor conductor of heat. Glass, therefore, tends to shatter or crack when heated or chilled very quickly because the surface of the glass responds to a change in temperature (by expanding or contracting) before the interior does.
Light is refracted (bent) as it enters or leaves a piece of glass. Perfectly clear glass is transparent to all wavelengths of visible light. Different kinds of glass vary in the amount of ultraviolet radiation or other forms of electromagnetic radiation they transmit. Colored glass is opaque to some wavelengths of visible light and transparent to others.
Glass has a smooth, nonporous surface and is resistant to chemical attack. These properties make glass useful for food containers and laboratory apparatus.