The refractive index indicates the extent to which a light beam is deflected when passing from vacuum into a given substance.
The relation between the turbidity value and the solids concentration of a suspension is influenced perceptibly by the light-scattering properties of the solid particles. The quantity of scattered light depends not only on the concentration, but also on the particle size and the ratio of the refractive indeces of the particles and the medium.
The more the particles differ from the medium (i.e. the more their refractive indices differ), the more light will be scattered by the particles. If there is no difference at all, no light will be scattered. This can be illustrated experimentally, namely by suspending SiO2 in liquids of differing refractive index and then plotting the resulting scattered light intensities against the refractive indices:
At the low point of the curve, there is virtually no scattered light at all and the suspension no longer appears turbid. At this point, the medium and the suspended SiO2 have the same refractive index (1.47). This method can be used to determine the refractive index of suspended particles.
The following table lists the indices for some common substances.
In actual practice the carrier medium is usually air (1) or water (1.33), and its refractive index is included in the calibration of the turbidimeter. The refractive index has a perceptible effect on the measurement only in cases where the substance being measured differs sharply from the calibration substance.