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About reddening

  1. Oct 12, 2004 #1
    Hello, all,

    I have just learned reddening today from my "Foundations of Astronomy" course. The teacher explains that it is due to blue light is more easily being scattered than other light in the dust. But she did not gave us the sub-reason for this... :rolleyes: Could anybody here give me a hand?

  2. jcsd
  3. Oct 12, 2004 #2
    The intensity of scattered light is inversely proportional to wavelength to fourth power. Thus the blue light, which has the smallest wavelength, is more easily scattered, and as the result the passing light looks reddish
  4. Oct 12, 2004 #3
    Violet is scattered more, but the human eye is more sensitive to Blue - so the sky appears Blue to us.
  5. Oct 12, 2004 #4
    But why not red?

    Well, thanks a lot guys!! However, there is still one thing that confuses me, which is according what drnikitin said, short wavelengh light is always more likely to be scattered than long wavelenght light, so the sky should be red rather than blue, or it is just because of the sensitivity of human's eyes??.. :smile:
  6. Oct 12, 2004 #5


    It can be understood this way......You know, that for EM waves, E=h\nu. where \nu is the frequency of the wave. Now scattering goes as inverse square E and hence as (\nu)^(-2). Hence the result...."Shorter wavelengths scatter more".

    Now, this means blue will be scattered more than red. Now, one can set an analogy with a tea party in which there are two persons, one is very extrovert and interact with almost everyone in his course, whereas the other just comes to the party takes his part of refreshment and goes. If you are an observer (say, a photographer) observing the party. Whjom will you encounter the most? It is for sure, the former wil win the race and the later may go unnoticed.

    Almost simillere thig happens. The red comes and goes almost rectilinear. So you will see red only if, it entered from the diagonaly opposite side and if you got a chance to have it fallen on your retina. But with blue, it comes through any direction, but due to continuous scattering feels up the whole space and you can't escape it falling on the retina.....so al the way you see bule blue and blue
  7. Oct 13, 2004 #6
    Everyone is neglecting the real thing, each one has his own explaination, might some be correct. But actually it is explained by the Rayleigh's Law of scattering, it is propotional to 1/(wavelenght)^4.
    This means shorter the wavelenght larger the scattering, violet has shorer wavelenght in the visible spectrum hence easily scatered but red do not this is the reason red signal are indicates danger.
  8. Oct 14, 2004 #7
    OK, I got it. Thank you so much for your vivid analogies...

    Because I thought the blue light has been scattered towards the outer space so that we shouldn't see it, but actually it is not.

    I am still thinking about why human eyes are more sensitive to blue light, which will be my next thread in the biology forum...
  9. Oct 15, 2004 #8
    Who said that the blue light is scattered towards the outer space, phenomena of scattering is diffrent from the reflection, silly.
    If a human eye is more sensitive towards the blue light than in the dark blue light will help you to see things easily, but it is not so red light is more effective forseeing in dark.
  10. Oct 17, 2004 #9
    I think that human eye is more sensitive to blue light. That is why blue light can be used in compound microscopes to see more details on the object. On the other hand red light is used to see in the dark for the reason that it scatters less.
  11. Oct 17, 2004 #10
    Do you know anything about resolving power of a microscope, it is inversly proportional to wavelenght of the light used. blue light have large wavelenght in visible spectrum, using it will decrease the resolving power of the microscope. And i never heard that blue light is used in microscopes usually it is sun light or light with the wavelenght near to thats of the sunlight.
  12. Oct 17, 2004 #11


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    Actually, blue light has a very short wavelength compared to red, which is quite long. It's the frequency of blue light that is large in comparison to red.
  13. Oct 22, 2004 #12
    thanks for the correction, i just mixed up frequencies with wavelenght.
    everything else is fine.
    Last edited: Oct 22, 2004
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