That's interesting. All of the UV light source manufacturers that I know of quote wavelength of light rather than frequency but your argument makes sense. I suppose the wavelength of light specified for each light source is actually an indicator of the frequency? I think I will have to look...
If the primary germicidal UV wavelength is 270nm and the secondary is 254nm and I have a UV light source in air to provide germicidal effects in water, would the wavelength of the UV light source need to be 360nm and 338nm respectively assuming the index of refraction for water to be 1.33?