Chromatic Aberration
What is Chromatic Aberration?
 

 

 
From the picture there are many "purple fringes" around the leaves.  This phenomenon is called Chromatic Aberration.  Chromatic Aberration occurs because each piece of glass has some amount of dispersion.  Dispersion results in the difference in the index of refraction with wavelength.  Index of refraction is:

where c is the speed of light in a vacuum and v is the speed of light of the medium.  Generally as the wavelength increases, index decreases since the index of refraction for blue light (shorter wavelengths) is larger than that of red light (longer wavelengths) as seen in Figure 4.  So when light hits a lens, the different colors will leave at different speeds therefore each will have different focal lengths, and hence light disperses.  Furthermore v is measured by a standard form known as Abbe's number:

nD is yellow sodium D lines (589.3nm), nF is the blue F line of hydrogen (486.1nm) and the red hydrogen C line (656.3nm).  To see an example of it please go here.

 

 
In order to measure Chromatic Aberration it is the difference between the focal length of red and the focal length of blue.  To fix chromatic aberration, it can be done with either an achromatic lens or by coating the lens.

 
 
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