<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 B1,2,3 and C1,2,3 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 |
|---|---|
| B1 | 1.15150 |
| B2 | 1.18584x10-1 |
| B3 | 1.26301 |
| C1 | 1.05984x10-2 |
| C2 | -1.18225x10-2 |
| C3 | 1.29618x102 |
Using these in the above equation produces the following plot for refractive index versus wavelength:
.