Whispering gallery modes occur at particular resonant wavelengths of light for a given droplet size. At these wavelengths, the light undergoes total internal reflection at the particle surface and becomes trapped within the particle for timescales of the order of nanoseconds. WGMs are named after the whispering gallery at St. Paul's Cathedral in London.
The light is concentrated near the circumference of the particle and can be assigned a mode number and a mode order. The mode number, n, tells us the number of wavelengths around the circumference of the particle, and the mode order, l, the number of maxima in the radial dependence of the electromagnetic field within the droplet.