For colour to the human eye, white light must fall on an object which absorbs some of the white visible radiation leaving you to see what is not absorbed.

For example a blue object is absorbing the red part of the white visible spectrum.{complementary colours} So, if you look at a blue object in red light, it will appear black.

 Reflection- coloured objects you see when white light falls on them- and you see what is not absorbed.

Transmission � e.g. a theatre coloured beam- where white light passes through a coloured filter. The filter absorbs the complementary colour and you see what is not absorbed- instead transmitted through the filter.

Lasers generate a specific colour only (not white light) and you see that specific colour. This is generally true of emissive sources, such as sodium lamps (yellow), mercury lamps (bluish white); they emit a range of specific frequencies, but not generally over the whole visible spectrum) etc.

units of electromagnetic radiation
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