By Sunil Bhardwaj


A beam of light entering a dark room lights up the dust particles floating in air. Similarly when a strong beam of light is concentrated on a colloidal solution the path of the beam is illuminated by a bluish light (and becomes visible when observed from the side). This phenomenon is known as Tyndall effect.

The scattering of light by the colloidal particle is responsible for this phenomenon. The intensity of scattered light depends upon the difference between refractive indices of the dispersed phase and dispersion medium. Since this difference is quite appreciable in case of lyophobic colloids, they show well-defined Tyndall effect. This phenomenon has been employed by Zsigmondy on as the basic principle for the construction of ultra A normal microscope cannot be used to locate colloidal particles.

It is this effect which is made use of in the test box in testing the purity of SO2 for the manufacture of sulphuric acid by the Contact process.