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White light

  1. Mar 22, 2010 #1
    why is the light from the sun called white light?????????
     
  2. jcsd
  3. Mar 22, 2010 #2

    Dale

    Staff: Mentor

    The word "white" means all frequencies in the visible spectrum. The sun emits all of those frequencies as well as higher and lower frequencies.
     
  4. Mar 22, 2010 #3
    but the sun glows on orange colour does not that mean that it is emitting light of orange colour
     
  5. Mar 22, 2010 #4
    Yes, and that can mean that (1) the receptors in your eyes respond more intensely to yellow/orange light than to other colours; (2) the Sun indeed emits MORE yellow/orange light than others; (3) or both.
    There are many other things to take into account if you want a more thorough understanding of that, as for instance the scattering of higher frequency light (sp. violet and blue) by the N2 and other molecules of the atmosphere (which gives us the blue of the day sky, incidentally), or the combined effect of the three different kinds of photochemical receptors in the retina.
     
  6. Mar 22, 2010 #5
    The sun emits more yellow than any other frequency, and the sun only appears red/orange in the sky because the atmosphere is blocking out the other frequencies more intensly.

    Plus, your eyes are most sensitive to green frequencies, which is why that color is usually chosen for night vision (sometimes it's white).
     
  7. Mar 23, 2010 #6

    russ_watters

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    The sun only looks red/orange when it is near the horizon - when it is high in the sky it looks yellow.

    And green isn't most used for night vision: red is.
     
  8. Mar 23, 2010 #7
    mmmmmmmmmmmmm this might sound stupid but is the light from the sun litarally white????
     
  9. Mar 23, 2010 #8
    The light from the sun contains a distribution of frequencies that we often refer to as "white". It gets confusing, especially in the world of photography.
    As stars go, it's classified more as "yellow" (class G), having its peak output in the green part of the spectrum corresponding to a surface temperature of around 6000K.
    Stars with surface temperatures of around 9000 to 10000K are termed "white" (class A, eg Sirius). Stars such as Spica and Rigel are even hotter and are described as blue-white class O or B.
     
  10. Mar 23, 2010 #9

    Dale

    Staff: Mentor

    The color that you see when you look at the sun is actually closer to yellow than to orange. That is because the shorter wavelength blue light is scattered by the atmosphere to a much greater degree than the other wavelengths. This is what makes the sky blue. The sun then appears yellow because white - blue = yellow.
     
  11. Mar 23, 2010 #10

    SpectraCat

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    No. The color of light refers to the wavelength of the photons, and there are no "white" photons. When all of the colors in the spectrum are present at equal intensities, then we perceive the result as the color "white". In physics, "white light" is loosely used to mean a few different things, but it typically means light having a broad frequency range covering most or all of the visible spectrum.

    Also, as others have mentioned, the light coming from the sun is not white, strictly speaking. It has its maximum in the yellow-green part of the visible spectrum, and actually contains higher intensities in the cooler part of the spectrum (green to violet), than in the warmer part (red to yellow). However when we look at the sun, we tend to perceive it as appearing in the orange-yellow part of the spectrum, due to the "blue sky" scattering effect that others have mentioned.
     
  12. Mar 23, 2010 #11
    Just to clear things up a little: The sun is still a "yellow" class star, even before the scattering of its light in the atmosphere makes it appear even yellower than it otherwise would be.
     
  13. Mar 23, 2010 #12

    SpectraCat

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    Yes, but AFAIK that just a "common" designation for stars in a particular mass/temperature range, and is a reference to the average maximum in the solar http://www.soultek.com/blog/uploade..._captured_by_new_solar_technology-735670.jpg". The appearance of the sun to the human eye is determined by the integration of the entire visible solar emission spectrum, convoluted with a "detector function", that depends on atmospheric effects, and the wavelength-dependent sensitivity of the human eye (or any other detector). I don't actually know what color the sun would look from space, but from the emission spectrum, I would guess that it looks green or greenish-yellow to the human eye.
     
    Last edited by a moderator: Apr 24, 2017
  14. Mar 23, 2010 #13
    Yes, sorry if I was not clear. The description of the sun as "yellow" is its conventional spectral type. Such stars would appear yellow or white to the eye.
    The light from the sun is (by definition) white, as it emits the complete spectrum of visible wavelengths that our human eyes have adapted to. Any object that reflects this spectrum completely appears white. However, our brains adapt to "non-white" sources and compensate such that we perceive an object as white even when illuminated by, for example, a tungsten bulb.
    Sorry for being a little misleading.
    This little diagram, somewhat simplified, may help with the spectral types and their emission curves. (Colours given are conventional colour and not apparent colour. Sun is class G)

    stellar.jpg
     
  15. Mar 23, 2010 #14
    That is a pretty neat chart. Now I know how to roughly estimate the temperatures of all the stars that I see...
     
  16. Mar 23, 2010 #15

    sophiecentaur

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    Anybody who has tried to match up the 'white' paint on a car will know that there are a whole lot of 'colours' that could be called white.

    btw, you can produce what will 'look white' without using a continuous spectrum of all visible wavelengths. Careful weighting of red, green and blue coloured lights (even quite narrow bands of colour) can produce a white that is indistinguishable to the eye from white. But that 'white' may not make coloured objects look the same colour as they would in sunlight. Hence "can I take it outside and see what it looks like?" when buying clothes.

    When you think about it, it's not surprising that it must be impossible to describe, fully, a whole spectrum with only the three colour, RGB, signals used in TV.
     
  17. Mar 23, 2010 #16

    Dale

    Staff: Mentor

    Aaaagh! (My mind recoils in horror as it accidentally recalls endless rings of swatches) :eek:
     
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