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

Why does black color asorb light?

  1. Dec 31, 2011 #1
    We know that any objects which absorbs all color of light is said to be black. But what is the underlying physics behind the color of the object that white light reflects all color and black color absorbs all color??
     
  2. jcsd
  3. Dec 31, 2011 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    I think you are looking at it the "wrong way around". There is nothing about "being black" that causes something to absorb light. Rather there is some chemical property (outer electron shell, etc.- often it is that the material is made of many different elements that have different "energy levels") that causes the material to absorb all wavelengths of light- and since little or no light is reflected, we call it "black".
     
  4. Dec 31, 2011 #3

    phinds

    User Avatar
    Gold Member
    2016 Award

    Also, black is NOT a color ... it is the ABSENCE of color.
     
  5. Dec 31, 2011 #4

    DrDu

    User Avatar
    Science Advisor

    The question is very difficult to answer in generality. The problem is that absorption and reflection are not independent from each other and the dependence is highly non-trivial.
    For example you can try to write with a coloured pen used to write on transparencies on a black sheet of paper. The color of the light reflected will be complementary to the colour of the usual colour of the pen. So with the exception of metals, strongly absorbing materials reflect most light where their absorption is highest. Also the reflection of an absorbing material increases with the angle of incidence. To get a good black "colour" you need a thick sheet of a substance which is not too strongly absorbing, so that reflection on entrance becomes unimportant.
     
  6. Dec 31, 2011 #5
    @HallsofIVy: I would like to know that chemical property which gives the characteristic to a black pigment to absorb all color...

    @phinds: Well, i too agree that black as such is not color but i would like to know why a particular color is so.??Why a white color is white??why does it reflect all color...What is the physics behind this.

    @DrDu: I agree with you statements but can you please elucidate on my query further?
     
  7. Dec 31, 2011 #6
    We don't know why some compounds reflect red and some reflect blue. We don't even know why glass is translucent.

    Fluorescence suggests an answer. This is when you, for example, shine a green light on a rock and it reflects purple. The explanation is that the light energy stimulates electrons to jump to the next quantum level, but this is not a stable state, so they fall back to their original state, releasing energy as they do so. This release is in the form of photons, which have neither the frequency of the originnal light or of the rocks usual (dull gray?) color.

    Perhaps you could look at dark-colored elements (carbon?) and categorize them, since elements are very basic things.

    Maybe black substances have total internal refraction, where they bend incoming light at a 90 degree angle, so that it seems to disappear. They act like a strong prism. Is that what the carbon atoms are "trying to do" in a diamond, which has excellent refractive possibilites?

    Carbon also has one of the highest melting points of any element.

    If a chemist were able to engineer new molecules to act like a 90 degree prism, the case could be proven.

    Use analogies to other forms of electromagnetic radiation. X-ray diffraction is used to analyze crystals, the DNA helix, etc. Do some materials totally absorb X-rays?
     
  8. Dec 31, 2011 #7

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Black and white is more what you see instead of material property. The lack of light is dark, and it "looks" black. The night sky looks black. A closed room with no light inside looks black. If you do not see any light coming from an object you see it black.
    Some objects emit light, and they are visible because of their own light. Other objects do not emit light but reflect the light of the sun or other light sources, and wee see the reflected light.
    An object can be made black if the light is not reflected from it. It is not enough if it absorbs light. The best absorbers are also the best mirrors and they look bright and shiny when illuminated. But fine metal powders look dark, as the light enters among the pores and reflects many times, and all times it suffers absorption, till all the incident energy is lost.
    "A black body" in Physics is modelled with a cavity inside a perfect absorber with a tiny hole in the front wall, where light can enter, but never finds way out.

    Destructive interference also can be used to make an object dark. Antireflection coatings are fabricated from materials which are transparent, by choosing proper refractive index and adjusting the layer thickness.

    People say that something is white if it reflects all radiation. But a mirror, although reflects almost everything, does not look white. If you smash the glass to very fine powder, it will look white. A surface should reflect light diffusely, in all directions, to have the "white" appearance.


    ehild
     
    Last edited: Jan 1, 2012
  9. Jan 1, 2012 #8
    As we know that energy can neither be created nor destroyed then the light absorbed by a blackbody has to leave the body in some form or the other.. If u say a black body just keeps on absorbing light then that means that the temperature of the blackbody will keep on increasing with time...There would be something happening with the light falling on it..What is that after absorption of light happens with EM rays..
     
  10. Jan 1, 2012 #9
    Well,, i like your analysis to the phenomena and would like to further probe into the query by asking that "Do u refer to a phenomena where the light absorbed by black light is absorbed in one direction and reflected in different angle". If there is some reference book or article which can state the occurrence then kindly let me know...
     
  11. Jan 1, 2012 #10
    It is certainly the point I kept telling a fellow student when he asked." If black colour reflects no light, then it is that no light enters our eyes; so why do we see it black?"
     
  12. Jan 1, 2012 #11
    Total internal refraction, never heard of that! Maybe my ignorance, but I am only familiar with Total Internal Reflection. As far as google says, there is no such phenomena.

    If you meant total internal reflection, even then you are wrong. It only happens when light travels from denser to rarer medium. Black substances are opaque, no light travels through them. If it could travel, they would appear transparent.
    What do you mean by a strong prism?
    Carbon atoms' arrangement provide such brilliant TIR in diamond only because it let in light for TIR to happen.
     
  13. Jan 2, 2012 #12

    Claude Bile

    User Avatar
    Science Advisor

    It is to do with how our brain interprets the information being received by the light receptors in our eyes. When it comes to colour, we have three different types of receptors (cones) that are each sensitive to a different range of wavelengths. The colour we perceive depends on how strongly each receptor is being stimulated. We perceive white when all three cones are stimulated the same amount. We perceive black when no cones are being stimulated.

    - There are a vast number of wavelengths we cannot see. Black objects still emit radiation - its just outside our limits of what our light receptors can perceive.

    - RGB and CYMK colour schemes work because they stimulate our entire visual palette.

    - Colour blind people usually cannot distinguish between red and green because their red and green colour sensors get stimulated by (nearly) the same wavelengths.

    - Different species have light receptors that cannot detect wavelengths outside our visual range. Many insects for example can see UV radiation.

    - Some species possess four colour receptors instead of the usual three, allowing them to perceive a greater visual palette than humans.

    Claude.
     
  14. Jan 3, 2012 #13
    @Claude Bile: Do u mean to indirectly say that black is also emitting a wavelength which is outside Visible range and occupies some other EM spectrum. If you are saying so, i feel that you are denying the real definition of Black. "Black is something that absorbs all color",..so if something absorbs all color how can it reflect anything..And as per the experiments done since time in memorial "Black doesn't reflect any thing..It does emit different rays as per its temperature"..
     
  15. Jan 3, 2012 #14
    A very well mentioned query which can be thought..If Black doesn't reflect anything then y do we see black...
     
  16. Jan 3, 2012 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    In a dark room you couldn't distinguish a white card from a red or a black one because there would be zero light of any wavelength coming off any of them. Under illumination from a source with uniform spectrum over the visible range (Sunlight is a fairly good example of this although, of course, it has a peak of brightness around the mid range of wavelengths) then you would see the white card as white and the red card as red. As a (perfectly / non reflecting) black card would reflect nothing then your eyes /brain would register this fact and you would call it 'black' - in the same way that we experience 'silence', an empty cup or, more recently (since we deiscovered how to use Maths) the number Zero. The lack of a quantity can be just as valuable information for our brains as the existence of it.

    The way we actually appreciate colours, learn to recognise them and call them 'agreed' names is more complex and has consumed GBytes of chat on this forum!

    @Claude Bile: Using the RGB or CMY systems does not, in fact, allow us to "stimulate the entire visual palette" but it allows us to cover a large 'central' part of it that is encompassed by the Primaries we use. You cannot, for instance, match ANY spectral colour perfectly by using RGB phosphors. (Try getting true, spectral Indigo, for instance) However, I can still appreciate just how enjoyable it is to watch good colour TV.
     
  17. Jan 3, 2012 #16
    /\ /\ /\

    This post is the answer.

    I've used a shadow analogy...we interpret a shadow as a dark area we can't see into because there's not enough light being reflected back to our eyes, but because we know that its CAST by something we know is blocking the light...we don't think of the shadow as having a "color"...we think of it as too dark to see into.

    If we see the same "color" though, without an object casting a shadow, etc, we instead interpret that as a property of the object, rather than as a lack of light.
     
  18. Jan 3, 2012 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    If you play with the colour selector on your computer you can go for any 'Hue' you like and reduce the Luminance until you get to the same experience of 'Black'. Similarly, if you decrease the saturation control (move the cross hairs to the centre), the colours all go to various shades of Grey. Bright Yellow will go to a pretty convincing White. In both cases, you cannot discriminate the colour information.
     
  19. Jan 5, 2012 #18
    Actually, getting true black is very tricky indeed. Even one of the darkest materials, this only absorbs 99.5% of light. So there is no black, only varying shades of grey.
    Essentially, what makes something dark is when the bonds and the electrons in them absorb the light by the very nature that light is electromagnetic waves, and electrons have an electric charge.
     
  20. Jan 5, 2012 #19
    Can you kindly elaborate? I couldn't get that.
     
  21. Jan 5, 2012 #20
    Sorry, I explained that quite badly. Light is an electromagnetic wave. Atoms and bonds are made up of electrons. Electrons have an electric charge (and can move around quite a bit, so can be affected by them.). So this means materials can absorb electromagnetic waves.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why does black color asorb light?
  1. Why does light move? (Replies: 42)

Loading...