Rainbow

Discussion in 'Chemistry' started by timojin, Sep 14, 2017.

  1. timojin Valued Senior Member

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    Very interesting observation and posting. Thank you ..
     
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  3. James R Just this guy, you know? Staff Member

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    No. As long as light enters the droplet, bounces off the back surface and then refracts back out at you, the exact geometry doesn't matter.
     
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  5. exchemist Valued Senior Member

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    I hope to God you do NOT revive that thread. I recall you advanced some very silly ideas about clouds, which you clung to in spite of explanations from several of us.
     
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  7. timojin Valued Senior Member

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    You are putting a restriction "droplet, bounces off the back surface and then refracts back out at you" What about between spherical shape and elliptical shape were the elliptical shape is 45 degree off ?
     
  8. timojin Valued Senior Member

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    They were silly to you but I am not satisfied with the explanation.
     
  9. James R Just this guy, you know? Staff Member

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    What about them? Why don't you suggest why that would be a problem, if you think it would be?
     
  10. timojin Valued Senior Member

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    I suppose for the sake of appearance there is no problem . My point is the incidence of light into the particle is distance A and the exit will be A + a for an elliptical shape and for a spherical incidence and exit is the same . And so the rainbow colors should be different ?
     
  11. Michael 345 Valued Senior Member

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    Your forgetting square droplets with black outside walls with only a microscopic pinhole for light to go in but not out
    If the light is absorbed inside would we see a rainbow if we cut it open?

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  12. James R Just this guy, you know? Staff Member

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    You only see the rainbow when the angles are just right to reflect the light back into your eyes. In fact, it's true to say that even two people standing next to each other see different rainbows; they each have their own set of water droplets that is producing their individual rainbow.
     
  13. superstring01 Moderator

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    While I was driving home --back to St. Petersburg, FL-- after Hurricane Irma, I saw a rainbow. I felt like Jesus was telling me something.
     
  14. Dinosaur Rational Skeptic Valued Senior Member

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    You can see a rainbow when it is not raining on you.

    Those who think otherwise should experiment with a lawn sprinkler.
     
  15. Dinosaur Rational Skeptic Valued Senior Member

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    BTW: Most rainbows are less than a complete circle due to being cut off by the horizon. Small ones can be created by lawn sprinklers.

    A circular rainbow is called a glory. In nature they are only seen from high altitudes like being mountaintops or near to a mountain top.
     
  16. DaveC426913 Valued Senior Member

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    An elliptical droplet still has the geometry of a spherical droplet. There will always be a section of the surface that is at the correct angle - it might just be a little narrower or wider.

    And it's not easy to get an elliptical droplet, unless it's falling rapidly, in which case your rainbow might be rather fleeting.
     
  17. Kittamaru Now nearly 40 pounds lighter. Staff Member

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    Ultimately, the restriction doesn't matter - water will, in nature, always attempt to form a spherical form due to the force of the water molecules attempting to pull towards one another. Excepting outside interference, there should always be some amount of refraction - the question, I would imagine, becomes one of intensity (is it bright enough to be visible to us)
     
  18. adianadiadi Registered Member

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    small droplets or even thin films
     
  19. exchemist Valued Senior Member

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    Aha, that is an interesting remark, because the mechanism behind the rainbow colours in thin films is different from that in rainbows.

    For general readers:

    Rainbow colours in thin films are the result of interference, which is more closely related to diffraction than to the refraction one gets in the water droplets responsible for a rainbow.

    You get partial reflection from upper and lower surfaces of the film. If the length of the path light travels within the film is close to 1/4 the wavelength of light (or odd multiples of that), you get destructive interference, so no light is reflected. Whereas if the path is close to 1/2 wavelength (or even multiples of 1/4) the interference is constructive and you get reflection. As white light contains a range of wavelengths, a given path length* will eliminate some colours and reflect others.

    This is not a refraction phenomenon.

    * I say path length rather than film thickness since you get most reflection when the light strikes the film obliquely, and the angle of incidence obviously affects the length of the path taken by the light between upper and lower surfaces. So the colours suppressed and reflected will be different at different angles of incidence.
     
  20. Seattle Valued Senior Member

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    Since we are talking about rainbows...many people don't realize that there are double rainbows and that actually every rainbow is a double rainbow. You just don't always notice this due to viewing conditions.

    The colors are reversed in the second rainbow from the first rainbow. This also contributes to not noticing the second rainbow since the second rainbow starts with purple (hard to see) and as you move away from the primary rainbow things generally begin to fade (due to lighting conditions) and you don't notice the rest. You can test this out with your garden hose as well.
     

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