A perfect whispering-gallery-mode microsphere's transmission spectrum is determined by its radius, polarization, refractive index, and the refractive index of the outside medium. This Demonstration shows how the spectrum (and free spectral range) changes around the 1400 nm (1.4 μm) wavelength range as the radius of the fused silica microsphere changes.
The resonance wavelengths are calculated by using the approximation of asymptotic expansion of the characteristics equation . Different colors denote spectral sets in the infrared region with different radial mode numbers (this Demonstration shows the first four sets). Different depths are for distinguishability (usually modes with smaller radial mode numbers have deeper transmission depth).
 S. Schiller, "Asymptotic Expansion of Morphological Resonance Frequencies in Mie Scattering," Applied Optics, 32(12), 1993 pp. 2181–2185.