Each term of the sum representing an absorption resonance of strength B i at a wavelength √ C i. For example, the coefficients for BK7 below correspond to two absorption resonances in the ultraviolet, and one in the mid- infrared region. Analytically, this process is based on approximating the underlying optical resonances as dirac delta functions, followed by the application of the Kramers-Kronig relations. This results in real and imaginary parts of the refractive index which are physically sensible. However, close to each absorption peak, the equation gives non-physical values of n = ±∞, and in these wavelength regions a more precise model of dispersion such as Helmholtz's must be used.