There are many properties of substances particularly of solids and liquids which depend upon the nature of the surface. Surface represents the boundary which separates two bulk phases. This is also called interface and is generally represented by separating the bulk phase by a hyphen or a slash. e.g. The interface of surface between a solid and a liquid may be represented as solid – liquid or solid/liquid interface. There is no interface between gases because they are completely miscible.The branch of chemistry which deals with the nature of surfaces and changes occurring on the surfaces is called surface chemistry. Adsorption on solid or on solution surfaces, colloidal properties, heterogeneous catalysis, corrosion, etc, are important surface effects which are useful to understand many physical and chemical properties of the substances. Therefore, the study of surface chemistry is of great importance not only from the academic point of view but also from the point of view of its important applications in industry, analytical chemistry and our daily lives.
When a strong beam of light is passed through a colloidal solution placed in a dark place, the path of light gets illuminated as a bluish light. This is known as Tyndall effect and is caused by the scattering of blue part of light by the colloidal particles. The scattering is caused if the size of particles is of the order of wavelength of light. The same effect is not observed when the light is passed through a true solution as the size of solute particles is too small to cause any scattering.
The cause of tyndall effect is the scattering of light by the colloidal particles i.e. these particles first absorb the incident light and then a part of this light gets scattered by them.Since, the intensity of the scattered light is at right angled to the plane of the scattered light is at right angles to the plane of the incident light the path becomes visible only when seen in that direction.Note that the same phenomenon is noticed due to scattering of light by dust particles when a beam of sunlight enters a dark room through a slit. The true solutions do not exhibit tyndall effect because the particles in them are two small in size and hence donot cause any scattering.