Chromatic aberration
Another property of light is the dependence of refractive index of a transparent medium to the wavelength of light. The relationship is dependent on the material (type of glass) and is empirically determined.

Given that Snell's Law, which predicts refraction, is based on refractive index, it follows that refraction of light is wavelength dependent.

Refraction is wavelength dependent
Snell's law
A prism produces a rainbow by dispersion: wavelength-dependent refraction. (graphic from Wikipedia)
The dependence of refractive index to wavelength for various glasses (graphic from Wikipedia).
The effect of Dispersion on image formation by simple lenses: Chromatic Aberration
When white light passes through simple lenses, dispersion results in light being separated into its component wavelengths (prism phenomenon). The result is a blurred image with color fringes.
When an object is on the optic axis, the chromatic spread is along the axis. This is called "Longitudinal or Axial Chromatic aberration".
When an object is off the optic axis, the chromatic spread at right angles to the axis. This is called "Lateral Chromatic aberration".
Achromatic Doublet
The method to overcome chromatic aberration is to couple two lenses with different Dispersion Indexes. Then, the chromatic spread of one lens will be offset by the refocusing of the other. The earliest lens pair that produced this phenomenon was the Achromatic Doublet invented by John Dollond (1706–1761) in the 1750s.

In the doublet, the positive lens is made of Crown Glass (a; V>50) and the negative, second lens out of Flint Glass (b; V<50).

A numerical description of dispersion of glass was defined by Ernst Abbe. This "Abbe number", "V", is defined as a function of the refractive index measured for three selected wavelengths of light:
D-, F-, and C-lines (589.2nm, 486.1, 656.3; Green, Blue, and Red).