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Black Light

Discussion in 'General' started by THEkorean, 27 Dec 2006.

  1. THEkorean

    THEkorean 42

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    this question has been bugging me for a while, i was wondering if it is possible for any kind of device to emit a black light (as in truely black i.e. shine it on a white wall and a black spot will appear). I know that black is like all the colors of the visible light spectum being absorbed.. but that bring up the question, how do projectors create a black screen if it emits light...
     
  2. TekMonkey

    TekMonkey I enjoy cheese.

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    magic, lol.
     
  3. iggy

    iggy Minimodder

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    dear sweet baby jesus.
     
    Last edited: 27 Dec 2006
  4. Constructacon

    Constructacon Constructing since 1978

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    Projectors don't create a "black" light. They just aren't projecting light at the areas of the screen that look dark. As projectors are used in dark rooms (to give the best picture) there are very few other light sources that can illuminate those dark areas of the screen.... hence they look black.
     
  5. dfhaii

    dfhaii internets

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    I have a black hole for sale, careful not to put your fingers in it though.
     
  6. Mighty Yoshimi

    Mighty Yoshimi Motormouth

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    is that not what we call the abscence of light? Does it not just create a silohette for black?

    And i think a black lens can allow the passage of light, not TRUE black anyway, as it would absorb all the light

    Some Nonsense about them

    black light actually turns out to be purple, or ultra violet, make your mind up! marketers :thumb:
     
  7. Ramble

    Ramble Ginger Nut

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    The original poster is talking about the colour black, while the poster above me is talking about black light (just another name for ultraviolet light).

    In response to your question, no, there is nothing out there because black light is no light, it is absense not just another colour. I suggest you read wikipedia if you want to learn more about light.
     
  8. Stuey

    Stuey You will be defenestrated!

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    Hey now, this isn't THAT foolish of a question! I actually thought about this too, about a week ago!!

    Technically, there IS "black" light. Human eyes cannot see infrared light, therefore it appears to be "black" to us. Although, it will not occlude the reflection of visible light photons so it's not like there can ever be a shadow-casting flashlight.

    Whip out a camera and aim it at an IR remote control. Press some buttons and you'll see the LED as detected by the camera.

    There are IR photons. The human eye sees zero intensity. Zero intensity = black. Therefore IR = black light. =)
     
  9. Sloth

    Sloth #yolo #swag

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    So the camera would pick up the LEDs as glowing black? That would be so awesome :rock:

    but who knows with pics.. the black might just be white light inverted by some leet photoshop shaman
     
  10. Stuey

    Stuey You will be defenestrated!

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    Nonono, the camera picks up IR LED light as blue. The human eye cannot detect it so you cannot naturally see IR light. I was basically twisting things around b/c I'm a smart ass. But that's ok.
     
  11. JCBeastie

    JCBeastie What's a Dremel?

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    I think you could acheive this with resonance. You'd need a a light with a waveform opposed to frequency of light you wish to negate, same principal behind noise cancelling but with light.

    Although lights annoying ability to be both particle and wave may render this impossible...

    If it would work you'd need a device to emit all the counter-frequencies as it would be very colour specific.
     
  12. ch424

    ch424 Design Warrior

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    As JCBeastie says, you could use superposition to create a black spot - but creating a mini black hole would be easier.
     
  13. severedhead

    severedhead What's a Dremel?

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    So instead of a light, a dark?
    I've thought about this a few times, would be cool, and massively confusing just seeing it!
     
  14. Moriquendi

    Moriquendi Bit Tech Biker

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    I dont think you could use superposition because 'white' light is incoherent, certainly from a black body source like a tungsten filament in a projector.

    What we see as black in a projected image is just the absence of reflected light reaching our eyes from that portion of the image.

    Moriquendi
     
  15. Nexxo

    Nexxo * Prefab Sprout – The King of Rock 'n' Roll

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    Actually, the question is not that dumb. Those of us who did not sleep through physics class undoubtedly remember the phenomenon of light interference. Shine a laser through two narrow slits at a screen and you get a nice zebra pattern of light and dark bands, as photons converge at wavelengths that are alternatingly in synchronous or inverse phase, thus reinforcing each other on cancelling each other out.

    So as JCbeastie suggests, in theory it would be possible to create a beam of darkness, if you can somehow manage to project photons on to a screen with a wavelength that is an exact match, but also in exact inverse phase with the photons that are already hitting that screen. In practice pretty much impossible, but in theory it can be done.

    Now on the: "How does a film projector project dark bits on a screen?". As Moriquendi says: it only looks dark in comparison with the lighter parts of the screen. Human vision is in fact crap at discerning absolute levels of brightness --it can only judge brightness of one spot relative to that of the surrounding area. So on a partially brightly lit screen, an unlit part will look dark in contrast.
     
  16. Guest-16

    Guest-16 Guest

    I cannot get this to work :/
     
  17. severedhead

    severedhead What's a Dremel?

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    I cannot get this to make sense!

    So in a very basic manner of speaking, if the beam is the exact wavelength and opposite phase as the ambient light it can, in theory, cancel it out to provide darkness?
    Just like a headset with 2 mics to cancel out ambient sound?
     
  18. Krikkit

    Krikkit All glory to the hypnotoad! Super Moderator

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    Seriousleh? Lool. :p
     
  19. Mighty Yoshimi

    Mighty Yoshimi Motormouth

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    Constructive and destructive interfearance, can be used for many applications, measuring the thickness of hair, theres youngs double slit experiment. etcetc.

    Same happens with soundwaves, set two speakers up in mono, and sit them about 3 metres apart, get them to play a constant tone. So the Set up is like this.
    .............................Speakers

    ................... / \
    /''''\ <- - 3Metres - -> /''''\

    l Stand about 2 metres away.
    \/

    -> Walk past them in the direction of the arrow (doesn't matter if you go from left to right or right to left, and the volume of the tone will vary as you get constructive and destructive interfearance.
     
  20. ch424

    ch424 Design Warrior

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    Nexxo, that's what I said - and the people at CERN (or JET, I can't remember) recon they can make mini black holes... so there's more hope down that road :D

    I thought the point of the laser shining though two slits thing was the wave-particle duality: even if you only emit one photon at once, though the system, you still get the superposition - weird! Though I think QED explains it....
     

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