Today I am not feeling so bright and I am certainly not venturing out of the house, but I did decide to undertake DDW’s January challenge. Today’s subject is white and I thought this approach might be appropriate for me, as a Chartered Engineer with a background in optics and lighting, among a number of other engineering topics.

Black is the complete absence of any colour, while white is the presence of every colour in an appropriate ratio. The human eye however has colour sensors (cones) which only detect red, green and blue, so that all colour perception is made up from the output of these three sensors. Orange light is perceived when a mixture of red and green light fall upon the eye. Yellow light is similarly perceived when a difference ratio of red and green light fall upon the eye, obviously red, green and blue light are perceived exactly as they are; this is known as additive mixing of the colours.

Artists among you may say that this is wrong, but this is because white light falling on a blue surface is blue because only the blue light is reflected from this surface. Yellow light falling on a surface is yellow, because only yellow light is reflected back, all other light being absorbed. When blue and yellow are mixed the yellow light is absorbed by the blue and the blue light is absorbed by the yellow, with only the narrow part of the spectrum, green, between yellow and blue not being fully absorbed; hence it is partially reflected and looks green; this is subtractive mixing of the blue and yellow.

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