Glass is a solid, brittle and usually transparent material, explains HowStuffWorks. It is amorphous, which means it is not made of crystals. The properties of glass can be varied by changing the materials used in its manufacture and the proportions in which they are used.
Many substances can be added to glass during the manufacturing process to change its properties. Boron is added to improve thermal and electric resistance, barium to increase the refractive index and lead to increase brilliance and weight of the glass. Glass is transparent to all colors of light in its clear form and can be made opaque to certain colors by changing its color via the addition of metallic oxides. Light can bend a beam of light. The bending of light when it passes from air to glass is called refraction, and occurs because glass is denser than air and it decreases the velocity of light when light passes through it. When a beam of light hits glass at a particular angle, light is bent far enough for it to be reflected within the glass in a process called total internal reflection. This makes glass an ideal candidate for making fiber optic cables, which transmit signals over long distances using the total internal reflection of light within the cable. Glass is a poor conductor of electricity and heat.