When a strong beam of light is passed perpendicularly through two solutions (1) true solution and (2) colloidal solution place against a dark background:
1.The path of light beam is not visible in case of true solution.
2.The path of light beam is visible (scattered) in case of colloidal solution and further it is forming a shadow (beam or cone) at the dark background.
This phenomenon of scattering of light by the colloidal or sol particles is called Faraday-Tyndall effect
The illuminated beam or cone formed by the sol particles is called Tyndall beam or Tyndall cone.
Faraday Tyndall effect in our Daily life :
1.Light beams from headlights of cars and motor vehicles on a dusty road.
2.Illumination of light beam from film projector in smoke filled theater's
3.Blue color (shorter wavelength) of sky is due to more scattered light of colloidal particles in upper atmosphere and the light transmitted to the earth is yellow or red (larger wavelength) in color.
This optical property of light is used to distinguish colloidal solutions from true solutions and also to depict the:
1.Size of colloid.
2.Shape of the Colloid.
3.Molecular weight of Colloids.
Determination of Molecular Weight of Colloid:
When a beam of light of intensity I is passed through a colloidal solution such that the incident light is scattered in all directions measured as Is. From this turbidity T can be calculated as fractional decrease in intensity of incident light when passed through 1 cm of solution.
At a given concentration of dispersed phase turbidity is proportional to molecular weight of colloid, provided the dimensions of particles are smaller than the wavelength of incident light.
A plot of concentration c on X-axis and H.c / T on Y-axis gives a straight line with a slope of 2B; the intercept on the H.c/ T is 1/ M, the inverse of which gives the molecular weight of colloid.
Determination of size, shape and structure of Colloid:
1.In the above experiment, when the molecule is asymmetric, the intensity of scattered light varies with angle of observation; such a data is used to predict size and shape of colloidal particles.
2.Use of electron microscope also reveals the structure, size and shape of colloid due to the high resolving power of electron microscope.
1.Quantification of this effect used to reveal the size, shape, structure and molecular weight of colloids.
2.It is used to distinguish colloidal solutions from true solutions.
3.To detect the presence of plant pigments or microbes like viruses and bacteria (colloidal size range) in solutions.
4.To study the colloidal aggregation and micellar formation.
1.Essentials of Physical Chemistry by B.S.Bhal, C.D. Thuli and Arun Bahl, 24th edition; Page nos: 653-654
2.Martin's Physical chemistry and Pharmaceutical Sciences, 5th Edition; Page nos: 475-477.