Is it possible for a light source to be anything other than white or a colour in the spectrum? For example, can one shine a brown light, or a pink one?
鈥� White light, as we see it, can be considered a mix of the three primary colours 鈥� red, blue and green 鈥� but sunlight emits all the colours of the visible spectrum (and beyond into ultraviolet and infrared, too). We can see this spectrum when using a prism or viewing a rainbow as the wavelengths are separated into individual colours.
There are many different sources of light, but nothing natural emits brown or pink light. However, placing a filter in front of a white-light source to selectively block certain wavelengths and allow others to pass through would let you transmit any colour you like, so then pink or brown light would be possible. That said, finding the right materials and coatings to make such a filter would not be simple.
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Alternatively, a red-green-blue combination of light sources can be used to make any colour you choose by varying the brightness of the individual colours. This principle, which is used in televisions and phone screens, would permit you to project a pink or brown light by attaching a projector to your laptop.
Richard Simons, Sainte-Agathe-des-Monts Quebec, Canada
鈥� Go to a theatre or some restaurants and you will see plenty of pink light. It鈥檚 just white with additional red. Brown light is more problematic. It鈥檚 actually a dim yellow 鈥� that is, yellow with additional black. Such a colour filter is available for theatrical lighting.
John Woodgate, Rayleigh, Essex, UK
鈥� Purple LEDs, which produce light using standard LED technology, are available commercially. Pink LEDs are also sold. These use a pink phosphor to convert blue or ultraviolet LED light into pink. Ultraviolet LEDs 鈥� as used in banknote forgery detectors 鈥� are now quite common. When used with suitable phosphors, they could produce any colour.
Richard Lucas, Hawley, Hampshire, UK
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This article appeared in print under the headline 鈥淣atural colour鈥�