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Does skin perpendicular to sun’s rays burn faster than if not perpendicular?

  1. Mar 3, 2010 #1
    Does skin that is perpendicular to the sun’s rays burns faster than skin that is closer to parallel to the sun’s rays? I was certain that it does but a very smart friend/engineer is certain that it doesn’t and was incredulous that I thought it did.

    I really would like to know the physics behind it if he is right. I haven’t been able to find an answer to this question online, because most of the information online is about the intensity of the sun relative to the angle of the sun through the atmosphere, versus the object on the ground changing orientation. If he is right, would it mean that if you want to collect the sun’s energy in a small area, it would be best to place collectors at close to parallel to the sun’s rays as possible without overlapping, sort of like louvers? I haven’t seen any solar collectors like that. Thanks!
  2. jcsd
  3. Mar 3, 2010 #2


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    The power density (or energy flux) is maximum when propagation is perpendicular. When exactly parallel, the sun's rays travel past and are never absorbed (at least if your arm is a perfectly planar surface). In between, the flux varies as sine of the angle.

    The center picture here
    http://en.wikipedia.org/wiki/Flux" [Broken]
    shows how the flux decreases with angle.

    Burns occur more rapidly with normal (aka perpendicular) incidence. That's why your nose, shoulders and sometimes feet burn at midday when the sun is overhead, while your chest, legs and arms (if they stay by your side) do not.
    Last edited by a moderator: May 4, 2017
  4. Mar 3, 2010 #3


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    Another way is to look at the tan/burn of someone who did not move. If tan/burn was not dependent on angle of incidence, the line between red leg front and white leg back would be knife edge sharp, despite being on a curved form.

    It's not. Red-to-white is well-graded around the curve of the arm or leg.
    Last edited: Mar 3, 2010
  5. Mar 4, 2010 #4
    Thanks so much for the replies. His view is that there is no definite line on a sun burn because the skin that was closer to parallel was actually moving in and out of the sun as the sun moved or the person moved, so the amount of sun burn is based totally on the time of the exposure, versus the angle of the exposure.

    Thanks again for your replies!
  6. Mar 4, 2010 #5


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    Certainly. No question that is a confounding factor (I was hoping you wouldn't think of it :tongue: ). Simply seeing a continuous gradient from red to white on the side of a leg does not conclusively point to incidence of light.

    Motion will produce a similar result, so will movement of the sun over time (at least in principle).
  7. Mar 4, 2010 #6
    And the same effect is why solar panels make the most power when perpendicular to the suns rays, and why windows absorb the most solar heat when the sun is roughly perpendicular to the glass surface....
  8. Mar 4, 2010 #7
    ...and why the earth has ice caps. And seasons. And is hot at the equator.

    What is the smart friend's explanation for these phenomena?
  9. Mar 4, 2010 #8


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    I missed the part that your "smart" engineer friend insisted this is so.

    I would like to know the following:
    Please ask your fiend what kind of engineer he is. Civil?
    Can you please ask him for a list any bridges or structures he has built, I'd like to know which ones to avoid.
  10. Mar 4, 2010 #9


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    Waitaminnit! Your friend is really on to something! He could make millions!

    See diagram.
  11. Mar 5, 2010 #10
    HAHA!YES!Thats very interesting idea! in your second graph, indeed wont much light will reflect, the efficiency can be maximised, however I m just wonder how much cost for one sun panel.

    Great idea:)
  12. Mar 7, 2010 #11
    Hello again. Thanks again for all of the information.

    What started my conversation with my friend was that I thought that the sun was strongest on my front/back (when standing) at about 6pm versus 1pm. He thought that due to the increased thickness of the atmosphere as the sun gets lower in the sky, that the sun is not nearly as strong, even on vertical surfaces, at 6pm.

    He still thinks that is the case, so thanks to some of your information I looked up some equations and tried to calculate it. Attached is an Excel spreadsheet and picture of what I came up with, including the sources. It does show the sun stronger at 6pm on vertical surfaces, but I'm not 100% sure that I did it correctly. Please feel free to correct me if you find an error on my calculations. I don't need super accurate equations. I just need a relative value between close to high noon and closer to sunset. Thanks!!!

    Attached Files:

  13. Mar 7, 2010 #12


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    Well, now you've thrown a monkey wrench into the works.

    Your question has changed to become:

    Which is a bigger influence on exposure: incident angle of sun to exposed surface? or thickness of atmosphere due to height of sun above horizon?

    That is a completely different animal!
  14. Mar 8, 2010 #13
    Yes DaveC426913, you are correct. As we were discussing about sunburn on the back/front of a standing person, we got into the disagreement about what I initially posted the question about, which was so black-and-white that I had to try to confirm one way or another. He now accepts that correction and says that was just a minor point of his view (it seemed major to me at the time), and says that his major point was/is the thickness of the atmosphere. Thanks!
  15. Mar 16, 2010 #14
    I think I will start a new topic to discuss sun exposure on the back/front at near sunset versus exposure on top of head at noon, since it definitely is a different topic. Thanks.
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