Prathamesh

Thank you people for all your replies. So finally we conclude that the reason for choosing 240 volt for domestic supply is quite arbitrary (Or if there is some specific reason, we have lost it in history).

I want to know the significance of the number "230" for the general household electric supply. Why "230" why not anything else?????

Thank You very much...I got it now

But practically speaking light is colourless. (We dont percieve light untill it is incident upon something, correct?) I am still confused. Consider the following: 1. When a beam of light is passed through a prism it disperses into its constituent colours. When the constituent colours are again combined by a prism we again get a white light (which is practically colourless) 2. When we paint the constituent colours of light on a disc in equal proportions and spin it we percieve the disc as white. Please distinguish between the above two phenomena.

Is there any difference in the term 'White' and 'Colourless' in Optics? Is a light ray colourless or white. In many books colourless and white means one and the same. But white is the colour of milk while water is colourless.