70 DISPERSE SYSTEMS 



suspension to the invisible solute. If the size of a particle is 

 decreased below 200/jt/u, it cannot be seen even by the most 

 powerful microscope made, or that could ever be made. The 

 particle is ultra-microscopic because its diameter is less than 

 half the wave-length of light. But, just as a beam of sunlight 

 renders visible innumerable specks of dust floating in the air, so 

 light may be diffracted by ultra-microscopic particles above a 

 certain diameter. 



Faraday-Tyndall Phenomenon. 



When a strong beam of light is sent through a rectangular cell 

 containing pure water, the beam may be rendered visible before 

 and after its passage through the water, but no cone of light is 

 seen in the water itself when viewed at right angles to the direction 

 of the light and against a dark background. If now a colloid be 

 dispersed through the water, light will be diffracted from the 

 particles in the water and the beam will appear in the solution 

 as a diffuse cone of light. This diffracted light is plane polarised 

 (p. 102), and is always produced when light passes through any 

 medium containing particles whose diameter is small in comparison 

 with the wave-length of light. 



White light is composed of waves of different lengths varying 

 from 760///A to 450/jt/x. When white light is scattered from a 

 surface instead of being reflected as in a mirror, it gives rise to 

 the sensation of white. Ice, in mass, does not appear white 

 because light is not scattered from its surface. If the ice is 

 powdered, light is scattered from the powdered surfaces and the 

 whole appears white. Crystallised copper sulphate appears blue, 

 but the light scattered from the surfaces of the finely powdered 

 crystals is white. The white colour of the lily or of white hair is 



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not due to the presence of a white pigment, but to the scattering 

 of light from the surfaces of innumerable minute air bubbles 

 embedded in the tissue. From this it follows that particles of 

 different sizes will scatter light of different wave-lengths. In 

 short, the colour of the scattered light may serve as an indication 

 of the size of the particle, provided the difference in the index of 

 diffraction between the dispersoid and the dispersant be kept 

 constant. 



The blue colour of the sky is explained by considering that the 

 fine particles of dust, globules of water, etc., suspended in the 

 atmosphere cause lateral diffusion of light of short wave-length 

 giving a blue colour, while the red rays are transmitted direct, 





