<math> n^2(\lambda) = 1 + \frac{B_1 \lambda^2 }{ \lambda^2  C_1} + \frac{B_2 \lambda^2 }{ \lambda^2  C_2} + \frac{B_3 \lambda^2 }{ \lambda^2  C_3} </math>
where B_{1,2,3} and C_{1,2,3} are experimentally determined Sellmeier coefficients. These coefficients are usually quoted for λ measured in micrometres.
The equation is used to determine the dispersion of light in a refracting medium. A different form of the equation is sometimes used for certain types of materials, e.g. crystals.
As an example, the coefficients for a common borosilicate crown glass known as BK7 are shown below:
Coefficient  Value 

B_{1}  1.15150 
B_{2}  1.18584x10^{1} 
B_{3}  1.26301 
C_{1}  1.05984x10^{2} 
C_{2}  1.18225x10^{2} 
C_{3}  1.29618x10^{2} 
Using these in the above equation produces the following plot for refractive index versus wavelength:
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