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Reflection and absorption of UV light

  1. Dec 14, 2015 #1
    Hi,I have a question when I was reading this website:
    http://www.webexhibits.org/causesofcolor/7F.html
    It explains how melanin works to protect us against UV light.
    From highschool physics course we know that black absorbs light while white reflects.
    I understand when melanin accumulates,more UV light would be absorbed and hence a better protection.
    However,if you think about it,white reflects all light so the effect would be the same as if all light is being absorbed(since nothing is gonna penetrate deeply into your skin.)But this is certainly a contradiction.

    Moreover,animals living in the arctic circle(e.g.arctic wolf,arctic fox,polar bears)have white fur.Despite of the fact that it's always night for half of the year,snow reflects 80% of the UV light.In contrast,a bituminous surface reflects 5-10% in a normal sunny day.So,following the chemistry of melanin,polar bears should have a black fur instead of white?What more funny is that for some creatures residing in the high mountains(e.g. bears),they have developed a black or brown fur to resist the increased UV radiation.But 80% of UV light is certainly a great amount ?Though there is less sun radiation on the Arctic circle.

    Hope someone can explain the "paradox".
    Thx :)
     
  2. jcsd
  3. Dec 14, 2015 #2

    DrClaude

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    Staff: Mentor

    An object is white because it reflects all visible light. That tells you nothing about what is going on in the UV. As light skin evolved so that northern humans could produce enough vitamin D, it means that white skin does allow the penetration of UV light.

    Fur protects the skin against UV light. The color of animal fur has nothing to do with UV light from the sun, but is all about camouflage.
     
  4. Dec 14, 2015 #3
    Oh,so you mean a white object only reflects the visible light.I thought it was applied to all EM waves coz a black body is defined to be an object which doesn't emit or reflect EM waves,so I thought that white behaves in an opposite way as a black body.Thx
     
  5. Dec 14, 2015 #4

    DrClaude

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    The term "black body" is very specific, and an idealization. Some black objects are very close to being black bodies, while many things painted black would not look "black" at frequencies other than visible light. Also, a black body does emit light, which depends on its temperature. Indeed, "black body emission" is a very important concept in physics.
     
  6. Dec 14, 2015 #5

    davenn

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    no the other way around
    We ( our brain) perceive the colour white BECAUSE all wavelengths within the visible spectrum are reflected
    The less of particular wavelengths that get reflected means that we will perceive a colour other than white

    and can you please put spaces after the punctuation in your text ... makes it easier to read :smile:

    Dave
     
  7. Dec 15, 2015 #6
    I have a question based on the black body radiation..:nb)
    According to wiki , a white body is an object reflects all radiations on it regardless of directions:
    https://en.wikipedia.org/wiki/Black_body.

    So,an idealized "white body" should be white in this sense,but an object appears white does not necessarily to be a white body?Take the fur of a polar bear as an example,it reflects all the visible light but not the UV radiation?
     
  8. Dec 15, 2015 #7

    DrClaude

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    Staff: Mentor

    Correct.

    Polar bears look black in UV light: http://it.stlawu.edu/~koon/polar.html
     
  9. Dec 15, 2015 #8
    Dark skin is resulted from the over-exposure to sunlight.Polar bears appear black in UV because it is absorbed by keratin in their fur.We all have keratin in our hair and epithelial cells,this is the "minimum protection against UV light". Hence, I guess melanin provides an extra protective layer on the skin for those living in the tropic areas?In other words,any creatures living above the 2 tropics should have a lower concentration of melanin(but same amount of keratin), and this accounts for their paleness?
     
  10. Dec 15, 2015 #9
    UV is not as easy to reflect as visible light, so instead we evolved melanin that will absorb UV. It's better to absorb the UV in the melanin than let it penetrate to deeper layers of skin where it can wreak havoc. It would be even better, perhaps, to reflect the UV, but evolution can only do so much.
     
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