Refractive index based measurements

A refractive index based measurement of a property of a fluid is measured in an apparatus comprising a variable wavelength coherent light source (16), a sample chamber (12), a wavelength controller (24), a light sensor (20), a data recorder (26) and a computation apparatus (28), by - directing coherent light having a wavelength along an input light path, - producing scattering of said light from each of a plurality of interfaces within said apparatus including interfaces between said fluid and a surface bounding said fluid, said scattering producing an interference pattern formed by said scattered light, - cyclically varying the wavelength of said light in said input light path over a 1 nm to 20nm wide range of wavelengths a rate of from 10Hz to 50 KHz, - recording variation of intensity of the interfering light with change in wavelength of the light at an angle of observation, and - calculating a said property from said variation.