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Why is visible light not harmful?

  1. Jun 6, 2015 #1
    [unacceptable source deleted]

    Microwaves have a wavelength greater than visible light and are harmful.

    Ultraviolet light has wavelengths smaller than visible light and is harmful.

    In what ways are the two harmful and why isn't visible light harmful in the same ways?

    I'm a total noob. Just wondering.
     
    Last edited by a moderator: Jun 6, 2015
  2. jcsd
  3. Jun 6, 2015 #2

    Simon Bridge

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    No light is harmful at the doses normally encountered in Nature - this is because we have evolved to cope with them.
    All light is harmful to some degree.

    Some light does damage to our bodies at lower doses than others for a number of reasons to do with how much energy gets delivered for the amount of light.
    You will understand that higher energy light is more harmful than lower energy light?
    There are also resonant effects which allow more energy to accumulate - like the one between microwaves and water.
    Microwaves heat the water in your body cooking you.

    These resonant effects are exploited in our eyes to make some light "visible".

    Note: iirc cellphone use was linked to a tiny increase in the risk of a rare kind of brain cancer but all other tests have come back negative ... any effect is less than the accumulated risk from just living on the Earth.
     
  4. Jun 6, 2015 #3

    Drakkith

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    There's two key concepts here. The amount of energy delivered to an object over time (known as power and measured in watts) and the energy per photon.

    A microwave oven is dangerous because it delivers around 1000 watts of power to an object. 1000 watts is 1000 joules per second. It takes 4.2 joules to heat 1 gram of water by 1 degree celsius. So my 1000 watt microwave can heat up 1 kg of water by 1 degree every 4-5 seconds. The dangers associated with microwaves comes solely from their ability to heat up body tissue if you are too close to a high-power transmitter or somehow manage to get a microwave to work with the door open (or if the shielding around the door is damaged). At the extremely low intensities associated with cell phones and wireless devices there simply isn't enough power to harm you. When the power is measured in millionths of a joule per second or less, and the absorbing body is on the order of 50-100 kg, it's simply harmless. You absorb FAR more energy just by standing in sunlight.

    Ultraviolet radiation is harmful because the energy per photon is high enough that a single photon can ionize the molecules in your body, which can destroy or mutate DNA, proteins, and other things. Microwaves have much less energy per photon, many orders of magnitude too low to ionize anything.
     
  5. Jun 6, 2015 #4

    Vanadium 50

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    As alluded to, like anything else, too much is dangerous. (You see this in foods too - too little iron, and you die. Too much iron, and you die) If you look straight at the sun, it's not good for you. Too much visible light will kill cells.
     
  6. Jun 6, 2015 #5
    Two phenomena are important. Power and resonance.
    High power is dangerous regardless of wave length. Lasers and microwave ovens are in this category.
    Natural light sources ordinarily do not have the surface brightness to harm us with visible light.
    Resonant radiation is dangerous even at low intensity.
    A single UV or higher energy photon can damage a body molecule.
    Our bodies can handle a certain level of damage but exposure at any level of intensity is a risk.
     
  7. Jun 6, 2015 #6

    anorlunda

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    Quite true, but a more complete answer would mention that total lack of visible light is harmful, maybe even fatal, to humans.

    Comparative risks, not absolute risks, are the watchwords.
     
  8. Jun 6, 2015 #7

    mfb

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    It is also highly impractical as visible light is great for orientation.

    A total lack of infrared radiation can be fatal as well, unless it gets compensated by intense visible light you would freeze to death.
     
  9. Jun 6, 2015 #8

    Simon Bridge

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    ... same with the lack of UV, needed to make vitamin D.
    OTOH: if we had evolved on a world without those things then their absence would not be a problem.
     
  10. Jun 9, 2015 #9
    An absence of UV light can possibly be tolerated if compensated by vitamine D pills.
    I doubt but do not exclude that such an approach is safer than just taking sunlight with moderation.
     
  11. Jun 9, 2015 #10

    sophiecentaur

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    I wondered whether I should shed some light on this question but decided against it.
     
  12. Jun 10, 2015 #11

    sophiecentaur

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    I am really sorry and, Dave, you should not encourage me. :smile:
     
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