The discussion centers on the physics behind why black objects absorb light while white objects reflect it. Participants explore the underlying properties of materials that lead to these behaviors, touching on concepts such as absorption, reflection, and the nature of color.
Participants express a range of views on the nature of color and light absorption, with no clear consensus on the underlying physics or the definitions of black and white. Multiple competing perspectives remain throughout the discussion.
The discussion reveals limitations in understanding the precise mechanisms behind light absorption and reflection, as well as the definitions of color. Some claims are based on assumptions that are not universally accepted.
ehild said: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 modeled 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
Pitagorass said: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?
phinds said:Also, black is NOT a color ... it is the ABSENCE of color.
Pitagorass said: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?
shashipoddar1 said:@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?
rktpro said: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?"
sophiecentaur said: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.
jetwaterluffy said: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.
rktpro said:Can you kindly elaborate? I couldn't get that.
An object's temperature does not relate to the reflected light from it, does it?sophiecentaur said:Remember, if it is not at zero K, there will always be radiation from it.
exactly this, EVERYTHING emits infrared radiation us, the earth, the sun etc, which is a wavelength not visible by us and is dependent on its temperature, the higher the temperature the more IR that is emitted.Remember, if it is not at zero K, there will always be radiation from it.
Johnahh said:if you were to put a black material in sunlight alongside a white or any other colour material for a period of time the black material would be hotter, as it absorbs light
Johnahh said:sorry, but for clarification it is black because it absorbs all incident light, and does not emit visible light, for a black body to emit visible light it would have to be at a high temperature.
i suggest you read this http://en.wikipedia.org/wiki/Black_body#Human_body_emission
it has some good points on black bodies.
Indeed. (I was wondering when someone might raise 'black as a perception' rather than as a 'fact'.)sophiecentaur said:The fact is that there is no such thing as 'pure black'.