The speed of a light wave is dependent upon the properties of the medium. Light travels fastest through a vacuum; slowest through solids. In the case of electromagnetic waves, the speed of the wave depends upon the optical density of that material. The optical density of a medium is not the same as its physical density or the mass/volume ratio. The optical density of a material relates to the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. The more optically dense that a material is, the slower that a wave will move through the material.

A beam of visible light is passed through a plexiglass container filled with water. If a beam of light comes in at a slight angle, describe the change in the path of the beam of light as it passes through the container, moving in from one side and then out the other.