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What makes the gas lamps light a certain color?

  1. Oct 13, 2012 #1

    Looking at the emission spectra of Mercury, for instance, how do the lines (I count 9 in the visible range: dark red, lighter red, orange, 3 yellows, green, purple, dark purple) superimpose to give light blue glow?

    My lab partner said that blue is the average of the wavelengths, such as when red and blue filters are put ontop of each other) would give green. But how does that explain white light, which can break down into basically every color. Wouldn't the average give green-yelow?

  2. jcsd
  3. Oct 13, 2012 #2


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    This has to do with the way the eye works. There are three kinds of visual receptors for color, which are sensitive to different wavelength ranges. They send signals to the brain, which combines them to produce the sensation of color. Googling on phrases like "biology of color perception" and "neurophysics of color perception" might give useful results.
  4. Oct 13, 2012 #3


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    As with all colours, white is just a name your brain gives to the combination of signals which your three, wavelength selective, sensors produce. 'Colour' is just in your mind. It so happens (evolution took care of it) that humans all more or less agree about the way that different combinations of 'primary' colours can be mixed to match the spectra of light from most sources / objects. Having learned the agreed or learned 'names' for colours and shades, we have a fairly good consensus about what to call a new colour (unless you are buying clothes with your wife - when all bets are off).

    Our colour vision is a fantastic system, based on a very crude analysis, for us to get 'just enough' information about our surroundings to be useful. But, as I have often said, the eye is not a spectrometer - because it doesn't need to be. It can spot shades of green - to recognise useful and dangerous plants, it is very perceptive of 'skin tones' - so it will recognise emotions etc etc but it can be fooled almost totally when looking at a colour TV picture that uses just three fairly pure primary display RGB colours. You really 'believe' what the TV display is showing you (colours, not programme content!).
  5. Oct 13, 2012 #4
    It may be 9 lines (depends on how sensitive is your instrument) but the blue and green are the strongest by far and they pretty much determine the color we perceive.
    You can see the relative intensities here, for example:
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