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B Blue Light

  1. Feb 19, 2017 #1
    For my astronomy class we were asked a question about a star that is 50,000K and it's peak wavelength. This was easy enough to find using Wein's law. The answer, however, is well below a human's threshold for visible light and we theoretically would not be able to see the star, but apparently it appears blue.

    I am just wondering why something with such a low wavelength, 58nm, would appear blue when we should not even be able to see it?
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  3. Feb 19, 2017 #2


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    We don't see the peak wavelength, as 58 nm is in the UV range. We are seeing the portion of the spectrum that falls in the blue area of the visible range. While the peak may be at 58 nm, there is still lots and lots of radiation at all the frequencies below (and some above) the peak.
  4. Feb 19, 2017 #3


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    For a given temperature, black body radiation is an entire distribution of frequencies with different spectral energy densities. So there is some radiation within the visible range. (see https://en.wikipedia.org/wiki/Wien's_displacement_law )
  5. Feb 19, 2017 #4
  6. Feb 20, 2017 #5
    I feel like I mostly understand. Essentially, the most waves happen at the peak, but there are other waves outside of that? Some of those waves are within the light spectrum, and according to the temperature of the star can appear blue - which has nothing to do with peak wavelength the problem just made them seem connected in how it was phrased?
    This is the graph given with the problem, and seeing as how the star is 50,000K there isn't a curve. I am assuming, however, that the curve would follow a similar pattern to the 6000K and because the frequency is on the order of magnitude of about 10^15, the light would appear blue.

    I just had a Eureka moment that I'm hoping someone can confirm for me.
  7. Feb 20, 2017 #6


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    Right. From your figure, as the temperature goes up there is more and more intensity within the visible range. At 50,000K there would be more blue light for us to see than at 6,000K. And the 50,000K curve would slope up through the visible range, so most of the light from it would be blue.
  8. Feb 21, 2017 #7
    Look at how the curves for 300 K, 1000 K, 3000 K and 6000 K have the same shape. The curves for higher temperature are higher and have their peak at higher frequency, but the shape is the same. So you can figure out what the shape of a curve for any other temperature should be.
    Now, notice how the low frequency ends of all curves are practically straight lines - same slope for all temperatures and frequencies far from the peak.
    If you want to know what the visible spectrum of say, 60 000 K black body looks like, then note how the visible spectrum is slightly under 10ˇ15 Hz. Look at how the curve for 6000 K looks like for slightly under 10ˇ14 Hz.
    That´s already the low frequency, practically straight line portion of the curve.
    Meaning that all black bodies whose peak frequency is well into ultraviolet have the same visual spectrum - a certain slope increasing blueward.
    It looks blue.
  9. Feb 26, 2017 #8
    IIRC there are quite a number of hot, blue stars known. Zeta Puppis, for example, has temperature of ~40000K.

    And yet, the photos of these objects... I would not describe them as "blue" (as I would blue paint or blue laser pointer's color). They are more like "blue-white", since they also emit a lot of energy in the rest of visible spectrum.
  10. Mar 2, 2017 #9


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    Blueish - like the sky. It has all visible wavelengths in varying proportions.
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