 Light is an electromagnetic wave that transports its energy at a speed of 300,000 km per second. This constant speed is how fast light travels in a vacuum and is referred to as the speed of light, or sea. When light encounters a medium such as air, water, or glass, it propagates through the substance at a net speed that is less than sea. This slowing down of light through transparent objects is a form of energy transport that involves the absorption and re-emission of the light wave energy by the atoms of the substance. Imagine that the electrons in an atom are attached to the nucleus by springs. The elasticity of these imaginary springs will determine how much the atom vibrates when acted upon by an incoming particle of light energy. When light impinges upon the atoms of a material, its energy is absorbed. This absorption of energy causes the electrons within the atoms to undergo subtle temporary vibrations. This transient vibrational action is transmitted from one atom to another until it emerges from the other side of the material at the same light wave frequency and speed. While these vibrations are only short-lived within each atom, overall these actions delay the transmission of the wave energy through the medium. Thus, while a light wave travels at a speed of 300,000 km per second through a vacuum, or the gaps between atoms, the absorption and re-emission process in different materials results in the net speed of the light wave to be less than the speed of light. In water, light travels at about 75% of its speed in a vacuum, or 225,000 km per second. In glass, light travels at 67% of its speed in a vacuum, so the light speed is 300,000 times 0.67 or about 200,000 km per second. Light travels very slowly through a diamond at about 41% of the speed of light, or 125,000 km per second, which is less than half the speed of light in a vacuum.