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B Why can't light be in black color?

  1. Jan 25, 2016 #1
    There are different colors of light that I have heard about but when I asked this question to my physics teacher she said I should ask the sun about it.So what do you guys think? Can't it be in black color maybe we are not able to see such a kind of light or maybe it exists it some other galaxy?
     
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  3. Jan 25, 2016 #2

    Grinkle

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    Sounds like your physics teacher is prompting you to do some thinking about concepts so you can pose your questions in a more addressable form.

    Black in the context of your question is, imo, an interpretation by the human brain of an absence of detectable light or also obtainable as a mixture of some pigments and in that context black is just as much a color as any other color our brains perceive. You can certainly buy a pen that is filled with black ink, and anyone you ask would agree that indeed the ink in the pen is black.

    These are not deep observations - that is why I think your physics teacher is wanting you to spend some more energy thinking about concepts.
     
  4. Jan 25, 2016 #3

    cnh1995

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    I'm no expert in optics but I believe colour is what our eyes make of the light falling on them. Black is simply absence of visible light. In space, everything looks black except for the blinking stars. Our eyes decide which colour it should be, when light falls on them. If no radiation from visible spectrum falls on the eyes, they treat it as 'black'. But for colour blind people, everything is black and white only. So, for human eye, 'black' is absence of visible light.
     
    Last edited: Jan 25, 2016
  5. Jan 25, 2016 #4

    davenn

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    just a note, when in space and observing the stars, they don't blink/twinkle .... it is when observing from earth that they are seen to twinkle
    due to atmospheric distortion as the starlight passes through it
     
  6. Jan 25, 2016 #5

    HallsofIvy

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    Start with your definition of "black".
     
  7. Jan 25, 2016 #6
    There are actually things which go by the popular name of 'black lights'.
    What they are in fact is UVA lights, florescent tubes usually, (UVA is the lowest frequency UV range, not dangerous).
    They're often used in discos and other entertainment events.
    While the lights themselves are barely noticeable, their UV light reflects intensely in the visible range from other objects which are illuminated.
    d801cc4b0bd0ceecb9fcd95ab3c42acc.jpg
     
    Last edited: Jan 25, 2016
  8. Jan 25, 2016 #7
    The colors we see are basically the colors that are reflected off of an object. So even if there was such things as black light, we wouldn't be able to see it because nothing reflects black since the color black is basically an absence of light.
     
  9. Jan 25, 2016 #8

    CWatters

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    If you were designing a new type of solar panel you would want it to absorb all of the light that lands on it because any light that is reflected is wasted. Scientists are developing coatings to do this based on nano technology. Such a coating would not only appear very black but it would also appear to have no texture or contours. Indeed it would be hard for your eyes to focus on it because there is so little reflected light for your eyes to process.
     
  10. Jan 25, 2016 #9

    fresh_42

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  11. Jan 25, 2016 #10

    cnh1995

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    Right! I should have said 'sky' instead of 'space':smile:!
     
  12. Jan 25, 2016 #11

    davenn

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    uh huh ...... no probs
    just wanted to make sure the OP wasn't unintentionally led astray :smile:

    It's been good on PF ... has helped me to improve in making sure I say things as accurately as possible
    Love the forum, it's been awesome for learn and expanding ones knowledge


    Dave
     
  13. Jan 26, 2016 #12

    sophiecentaur

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    A very good 'black' can be obtained by using a hole with a box behind it, lined with black velvet. Any light entering the hole (form any direction) will hit the velvet at the back and be mostly absorbed. Any light reflected off the back will be further absorbed by the velvet on the other sides. Hence no (or extrememly little) light will emerge from the hole. The hole will look much blacker than the blackest / most matt paint you can apply to a surface. It's quite impressive to see.
    The method is (or used to be, in analogue days) used for setting up the 'black level' on TV cameras, in brightly lit studios. Circuits and devices are more stable these days and don't need to be calibrated so often so it's something you probably don't need in a studio all the time.
     
  14. Jan 26, 2016 #13

    Svein

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    The box doesn't even have to be lined with something black. I remember my first course in optics at the university (more than 50 years ago, by Jove) where the theory was explained. We were then shown such a box, the hole was extremely black. When the lecturer opened the box, it was covered with mirrors on the inside!
     
  15. Jan 26, 2016 #14

    Merlin3189

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    Just to join the pedants, can I point out that reflected UV light is still UV and just as inconspicuous. Look in a mirror. Look at many white surfaces.
    Your intense "reflection" is actually caused by fluorescence not reflection. The UV is absorbed and visible light is emitted. The reason that white fabrics and some papers often do this, is because they are treated with "optical brighteners". These chemicals are intended to absorb the normally invisible UV in natural light and convert it into visible light, making the fabric appear "whiter than white".

    But Hallsoflvy has it - first decide what you mean by black. Some people here are using the definition I prefer - the absence of light. By that defn black can't contain any other light.
    There is also the psychological notion (I find it hard to call it a definition, without further qualification) of something which looks black. In that case of course black can contain any or all colours. For example looking at a blackboard you probably see much the same colours as when you look at the white chalk - just not quite so much of them (but a lot more than you might think.)
     
  16. Jan 26, 2016 #15

    fresh_42

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    Couldn't it simply be defined as 0°K?

    Edit: I've added the '°' although I didn't like it, just to distinguish it from 'o.k.'
     
  17. Jan 27, 2016 #16
    Does light have a colour until it interacts with an object?
    Can we see it as a laser beam or is it the small objects within the beams focus that interact with the radiation and glow as colour making it visable.
     
  18. Jan 27, 2016 #17

    jbriggs444

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    Light has a spectrum whether or not we can see it. Shine a ruby laser into space and we can say that the beam is red, even though it may never be seen. No need to make things complicated.

    "If a tree falls in the forest, does it make a sound? Yes -- the bigger the tree the bigger the sound" -- Jim Ignatowski
     
  19. Jan 27, 2016 #18

    fresh_42

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    Yes. But the term 'color' is already a measurement (by our eyes). So before we define 'black' or 'black light' we should not forget to define 'color'.
    I've always asking myself whether we both 'see' the same color if we say 'it's red' and how can we know?
     
  20. Jan 27, 2016 #19

    HallsofIvy

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    It depends on exactly what you mean by "color". Light has (a range of) frequencies and wave lengths, that determine its color, before it interacts with an object.
     
  21. Jan 27, 2016 #20
    I agree with that correction.

    Wavelengths in the region of around 650nm are 'red'.
    Two different people (without visual impairment) will agree that that is red.
    Different instruments can determine the exact wavelength.
    Comparing experiences of 'redness' isn't really within the domain of science, and I guess most philosophers would pass on that one too.
     
    Last edited: Jan 27, 2016
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