Optical Aberrations
There are five types of aberration that can degrade the performance of any optical instrument.  These are: Chromatic Aberration, Spherical Aberration, Astigmatism, Coma, and Dog Biscuit.
Chromatic Aberration occurs because violet light has a shorter wavelength than red light, and therefore comes to a focus before the red light does. In this regard, the lens is behaving like a prism, spreading its spectrum along the lens or mirror's principle axis.  This error is partially corrected by the use of a "doublet" lens consisting of a double convex converging front element made of crown glass, cemented with transparent Canadian Balsam to a plano concave diverging element with a matching curve.  Such a doublet is called an "Achromatic Lens," or "Achromat" for short.