View Chromatic Aberration
Chromatic Aberration is an optical artifact that is caused when light passes through lenses onto a recording substrate, such as photographic film or digital photo-sensors. Chromatic aberration is visible at object boundaries, and presents as a subtle shift of colors, where none are actually visible in reality. It results when differently colored parts of light (depending on the wavelength) are refracted with varying intensities within the different internal elements of the lens (the lens is actually several lenses, either concave or convex, arranged in a cylindrical housing). For the most part, photographers and cinematographers work hard to mitigate this artifact, though it can be used as a design element.
In CG work, however, it is the duplication of these real world artifacts that programmers strive for in creating digital imagery, as the presence of these artifacts can actually enhance the believability of the artificial image. Note, however, that the methods for calculating chormatic aberration at render time is expensive in terms of render time.