Explain the Tyndall effect.

(N/A) If a homogeneous solution placed in the dark is observed in the direction of light,it appears clear. If it is observed from a direction at right angles to the direction of the light beam,it appears perfectly dark.
Colloidal solutions viewed in the same way may also appear reasonably clear or translucent by the transmitted light,but they show a mild to strong opalescence when viewed at right angles to the passage of light,i.e.,the path of the beam is illuminated by a bluish light.
This effect was first observed by Faraday and later studied in detail by Tyndall and is termed as the Tyndall effect. The bright cone of light is called the Tyndall cone.
The Tyndall effect is due to the fact that colloidal particles scatter light in all directions in space. The scattering of light illuminates the path of the beam in the colloidal dispersion.
The Tyndall effect can be observed during the projection of a picture in a cinema hall due to the scattering of light by dust and smoke particles present there. The Tyndall effect is observed only when the following two conditions are satisfied:
$(i)$ The diameter of the dispersed particles is not much smaller than the wavelength of the light used.
$(ii)$ The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.
The Tyndall effect is used to distinguish between a colloidal and a true solution. Zsigmondy,in $1903$,used the Tyndall effect to set up an apparatus known as an ultramicroscope. An intense beam of light is focussed on the colloidal solution contained in a glass vessel.