Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Eppyfone on 18/02/2009 22:37:44
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So, we know that if you shine white light through a prism it will split into the colours of the rainbow. We also know that Orange can be made by mixing Yellow and Red. So if we shine Orange light through a prism, what will happen? Will it split into Red, Orange and Red?
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So if we shine Orange light through a prism, what will happen?
Will it split into Red, Orange and Red?
It all depends... [;)]
If it were monochromatic (single wavelength) orange then then you would just get a single pure orange line out. An example would be the older-style "low pressure" sodium streetlights. (Strictly this is two very close orange wavelengths at 589.0nm and 589.6nm).
But you could make exactly the same perceived colour from a whole family of different spectra comprising wavelengths from about 550nm (green) through to very deep red (700+nm). This could be a pair of monochromatic colours in a correct brightness ratio, or a continuum.
Consider the CIE chromaticity diagram about 1/3rd of the way down my webpage at http://techmind.org/colour/ (there's also a larger copy you can download)
You can make any (perceived) colour on there by taking any mixture of "primary" sources of different colours as long as their weighted average can be made to coincide with the required colour. This means that if you draw a polygon around your two, three, or more primary/source colours then your target colour must lie within the polygon - then by the correct weighting of brightness of the sources you can make that colour.
Clearly you can make the same "colour" from many different spectral "recipies". Colours which look the same but have different spectra are called metamers. Or a pair of spectra which look the same colour are said to be metermeric.
For my orange streetlamp example, the orange lies at about (0.57,0.43) which is close to the edge of the locus. For a multi-chromatic metamer you still require the weighted average to lie close to that edge - wavelengths longer than the 550nm all lie close to a straight line, so a suitable average will lie on the line. If you had much energy at wavelengths shorter than 550nm then you'd tend to desaturate the colour somewhat. See the description on the webpage of what the diagram showns and how it "works".
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So, we know that if you shine white light through a prism it will split into the colours of the rainbow. We also know that Orange can be made by mixing Yellow and Red. So if we shine Orange light through a prism, what will happen? Will it split into Red, Orange and Red?
It depends on how you *actually* made it. If it's monochromatic, it won't split, if it's made of two monochromatic wavelenght of red and yellow, it will split in these two, if it's made of a continuous range around orange, it'll split in the single wavelenghts of this range, ecc.
Techmind had already exhaustively answered you, but I thought to add just this little clarification.
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You may find this kitchen science useful
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/colours-in-cds/
I used a CD instead of a prism, but it is the same general principle
And
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/why-sodium-street-lights-make-things-look-orange/
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I used a CD instead of a prism, but it is the same general principle
and I reckon a DVD is even better than a CD.
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I am intrigued to learn that you can use a DVD as a diffraction grating, I recall reading as a boy about the elaborate and expensive machines for making these gratings now we can all have one.
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That's technology for you. I remember being shown a hologram in 1966 and almost no one at Uni really knew how it worked. They could only make them photographically. As you say, diffraction gratings used to cost a fortune.
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http://www.answers.com/topic/theory-of-colours
:)
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That's technology for you. I remember being shown a hologram in 1966 and almost no one at Uni really knew how it worked. They could only make them photographically. As you say, diffraction gratings used to cost a fortune.
and now they're available on ebay for a couple of pounds. Cheap enough that I can be relaxed about my kids playing about with them without worrying about them breaking them.