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B Black colour and light

  1. Jul 3, 2017 #1
    In the visible spectrum VIBGYOR, there is no black colour.So, what do we percieve as 'black' ?

    Another of my queries is that when dispersion takes place there is a change in wavelength but not so in case of frequency. But they are related inversely. So, why does this happen?
     
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  3. Jul 3, 2017 #2

    anorlunda

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    Black is the lack of light. White is the superposition of all visible colors.

    Just think of a black-and-white picture. The dark parts are black and the bright ones are white, medium intensities are grey.
     
  4. Jul 3, 2017 #3
    What do you mean by that? How is light absent? Is it something related to absorption?
     
  5. Jul 3, 2017 #4

    sophiecentaur

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    Bright / dark / black relate to intensity and not to wavelength or frequency (or chrominance).
     
  6. Jul 3, 2017 #5

    anorlunda

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    Go into a sealed room and turn off the lights. What color do you see?

    Yes, that's one way. A surface that reflects no light to your eye is perceived as black.
     
  7. Jul 3, 2017 #6
    So, someting absorbing all light is percieved to be black?
     
  8. Jul 3, 2017 #7

    sophiecentaur

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    Yes
     
  9. Jul 3, 2017 #8
    The frequency part of the question does not relate to black.
    Thanks for the clarification .
    And about the second part ?
     
  10. Jul 3, 2017 #9
    In your second part I would guess you misuse the term "dispersion". Dispersion means that the speed of the wave through a medium depends on frequency. You also can write velocity of wave as a function of wavelength. But dispersion does not mean a change in either frequency or wavelength. The speed in the medium is different than in the air and so the wavelength will change while propagating through that medium. The frequency does not depend on the propagation speed, is a characteristic of the source of the wave. But this happens even in a medium without dispersion.
     
  11. Jul 3, 2017 #10
    Yes.
    I tried to mean refraction.
    Thanks
     
  12. Jul 3, 2017 #11

    sophiecentaur

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    Frequency and intensity are independent. Intensity does have an effect on perceived color, though. You can see a yellow area and a brown area in the same scene. They can both have the same chrominance co ordinates. Our color perception falls off as light level drops and our vision becomes monochrome at very low levels. It's a bit of a grey area. ( pun intended)
     
  13. Jul 3, 2017 #12
    It's true frequency and wavelength are inversely related, but in refraction the wavelength and the speed of the wave change, the frequency doesn't change.
     
  14. Jul 3, 2017 #13

    Drakkith

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    Well, perfect absorbers do not exist. Typically an object with a "black" color absorbs a much larger percentage of incoming light across most of the visible spectrum compared to surrounding objects, though it never absorbs all of it. I am currently surrounded by about a dozen different objects that I would call "black" and each one actually looks slightly different than the others.
     
  15. Jul 3, 2017 #14

    russ_watters

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    Or not having them created to begin with. Consider a white movies screen.
     
  16. Jul 5, 2017 #15
    Why does the frequency not change?
     
  17. Jul 5, 2017 #16

    jbriggs444

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    Why does the frequency not change when a wave enters a medium like a prism and thereby speeds up or slows down?

    Because every time a wave crest arrives at the boundary the wave crest passes through and proceeds past the boundary. The frequency at which crests arrive is equal to the frequency at which crests proceed.
     
  18. Jul 5, 2017 #17

    sophiecentaur

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    Black is not a particularly quantitative thing. It is not 'less than 10 photons arriving per nanosecond'.
    The doorway into a bar can appear black at noon but, with the same lighting inside, it can be the brightest object in the street on a moonless night.
     
  19. Jul 5, 2017 #18
    I'm just curious, isn't a black hole a perfect absorber beyond the event horizon? Further, would it be accurate to state that the only known existence of "perfectly black" that we know of at this point is inside a black hole's event horizon as perceived by someone outside it? Or is this negated by the existence of Hawking radiation which should be visible in the IR range?

    I'm sorry if these are fairly stupid questions; I'm fascinated by the concept of black holes but don't know much about them.
     
  20. Jul 5, 2017 #19

    jbriggs444

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    Wiki can answer this. If one considers a black hole as if it were a "black body"...

    "A black hole of one solar mass (M) has a temperature of only 60 nanokelvins (60 billionths of a kelvin); in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5×1022 kg (about the mass of the Moon, or about 13 µm across) would be in equilibrium at 2.7 K, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb and thereby lose mass.[11]"​
     
  21. Jul 5, 2017 #20
    Another example: a light string attached to a heavier one. If you start a wave train in the lighter string, as each crest meets the heavier string it will become a crest there, so the frequencies are the same in each string. But for equal tension in each string, the wave in the heavier string is slower and the wavelength there is smaller than in the lighter string.
     
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