[wizakor]

I read in my textbook that colloidal solutions appear clear or translucent in transmitted light but show mild to strong opalescence when viewed at right angles to the passage of light, that the path is illuminated by a bluish light... wiki says opalescence is property that the sol appears reddish in transmitted light and bluish in scattered light... but i have not been able to see it practically... as in.. a good image or video showing this difference in colours so that i can visualise it properly... So can you help me in visualising the actual colour change?

[FreeTheBee]

A quick youtube search yields this video,
http://www.youtube.com/watch?v=E2ULbn7Uxsk

The difference in observed colour does not show here since a red laser is used. You would need a light source that contains a spectrum of colours and not just one.

Light scattering depends strongly on the wavelength of the light. The shorter the wavelength the stronger the scattering. So, blue light scatters more strongly than red. You can see this effect by looking at a blue sky and a red sunset. Consider the air around us as a colloidal solution (aerosol) and the sun as the light source. Looking up at the sky is like looking at the colloid solution from the side. Then you observe the blue light that was scattered. Looking at the sunset is like looking in the direction of the light source. Since light on the blue part of the spectrum was scattered, mainly the red part remains and the sunset appears red orange.

I hope it is not too confusing an explanation