Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black ?

  1. Aug 2, 2008 #1
    How does black absorb all the wavelengths ? & why does the reflection capacity of a transparent glass increases if black is put behind it .
  2. jcsd
  3. Aug 2, 2008 #2
    It's just an illusion, because you can not see what behind the glass then you think it reflects more. the refection capacity must be the same.
  4. Aug 3, 2008 #3
    Something that looks black may only be absorbing the visual spectrum. Fire some radiowaves at it and they might just go straight through it, making it transparent.

    In other words if our sight was adapted to see radio waves instead of the visible, the world would look a very different place, many non-metalic objects would appear transparent, and the sky would be constantly lit up with radio and TV signals bouncing around.
  5. Aug 3, 2008 #4
    Here is an interesting article on parakeets vision and how they use it

    http://www.lisashea.com/petinfo/articles/bird_sight.html" [Broken]

    A weird question.. I know that we can use equipment to see in infrared, but the image just uses our perceptible spectrum to give an image of what that would look like. Meaning that the equipment shows us a picture using visible red which isn't the actual color of infrared. Are those waves on the non-visible part of the spectrum colors that we can't comprehend? Do other animals see colors that we don't (and can't) have in our box of Crayolas?
    Last edited by a moderator: May 3, 2017
  6. Aug 3, 2008 #5
    To the original poster: I think you need to look at the problem the other way around: Black appears to us as black because it absorbs all wavelengths. There's not really a reason for 'black' things to do this, it is a fact that they do, and that makes them appear to us as black.

    The reflection of the glass is not increased if you place a black object behind it, this is merely a visual illusion (which I believe works equally well with all other solid colors). If there is no black object but a variety of objects behind the glass, it is hard for our eyes to focus on the reflection in the glass and the objects behind the glass simultaneously. If you try really hard however you can focus on the reflection and you will see that it is just as visible as it would be with a black object behind it.
    With the black object behind it, there is nothing for your eyes to focus upon behind the glass, so our natural reaction is to focus on the reflection, making it seem more visible.

    To 'blipped':
    The waves on the non-visible part of the spectrum can be regarded as colors, but I would like to see a definition of 'color' first... I think color is just what appears to us as the visible spectrum. We cannot see infrared or ultraviolet for example, but if we could see it there is no telling how it would look. We might have called infrared red and ultraviolet violet...

    Yes, many animals can see more wavelengths than us as far as I know...
  7. Aug 3, 2008 #6
    Thought experiment...not really a physics questions though.

    We see color in relation to the object it is on, like a leaf or dirt, etc... Our brain has the ability to process these colors because our eye is able to take them in. If we bypass the eye and send signals of an image in ultraviolet light how would our brain handle it?
  8. Aug 4, 2008 #7
    I'm not really an expert in brains lol, but I would guess that that depends fully on which type of signals we send. Red colors send a different signal than blue colors for example. If you send the UV light as blue color signals, we see blue light.

    You might want to send the UV light as 'UV color signals' but what would these be? I think it would be possible to send red light signals since we know what red is and we can see if the signals actually produce a red light. Since we don't know what UV light looks like I don't think we can send signals and then expect to see UV light...
  9. Aug 4, 2008 #8
    The "visible spectrum" is not the same across all "eye-bearing" species, or even within a same species class.

    For example, some insects visually perceive UV, whereas other creatures have a heightened visual IR.
    It is simply what nature has decided to vary the type of chemicals in the eye sensitive to specific wavelengths for survival, adaptation and/or predatory purposes.
  10. Aug 4, 2008 #9
    Also, the human skin has very sensitive IR capabilities. This is mostly due to the need to alarm the person of environmental changes in temperature which could be damaging or fatal.
    Why IR(or UV) capability is not visual in humans is unknown, but I suspect that it was not required for visual clues for feeding, and might actually immensely complicate human interaction with the environment due to "too much visual information"

    As you know, we have devices that can augment this when needed, such as IR imaging cameras and such.
  11. Aug 4, 2008 #10
    With regards to "black" put behind glass scenario, it is an "illusion" of sorts, as said before.

    If the glass would have a anti-reflective coating, it would not "mirror" regardles of how much black was on the other side.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook