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Why the black objects absorb more light and heat from the other color objects?

by Physicsissuef
Tags: absorb, black, color, heat, light, objects
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Physicsissuef
#1
Jan24-08, 06:06 AM
P: 908
Why the black objects absorb more light and heat from the other color objects?
What is the structure of the black color?
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pixel01
#2
Jan24-08, 07:23 AM
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Black objects absorb more lights than other colored so they appear black ! After all, things absorb more light and heat will appear more black.
Why they absorb more light and heat depends on the electron structure of the material.
Andy Resnick
#3
Jan24-08, 07:46 AM
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Hang on... just because and object is black (highly absorptive) in the visible just not mean it is also black in the infrared. Snow is black in the IR region, for example.

How efficient an absorber a dielectric (or any material) is depends on both microscopic bulk properties (atomic/molecular absorption) but also on surface structure- textured surfaces can appear black under some conditions.

DaveC426913
#4
Jan24-08, 08:38 AM
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Why the black objects absorb more light and heat from the other color objects?

Quote Quote by Physicsissuef View Post
Why the black objects absorb more light and heat from the other color objects?
What is the structure of the black color?
Without complicating the matter too much:

Black objects are black because almost all the light that falls on them is absorbed into the material. Little or no light is reflected back toward your eye, therefore you see black. That light that is absorbed ultmately becomes heat.

White objects are white because almost all the light that falls on them is reflected by the material. Because all the light is reflected back toward you eye, you see white. Little light is turned into heat.

Red objects absorb light at frequencies other than red and tend to reflect frequencies near red. They don't get as warm as objects that are black.


The nature of an object's colour has to do with the molecules and their electrons. Electrons in different states absorb different frequencies. When we build things - and especially when we paint them - we choose materials that contain molecules with very specific reflection frequencies. For example, Phthalo green - a paint colour - has very stable molecules in it that reflect light in a very specific, predictable band of green. Lamp black - another paint colour, has molecules that absorb ALL frequencies of light.
TVP45
#5
Jan24-08, 12:13 PM
P: 1,127
Quote Quote by Andy Resnick View Post
Hang on... just because and object is black (highly absorptive) in the visible just not mean it is also black in the infrared. Snow is black in the IR region, for example.

How efficient an absorber a dielectric (or any material) is depends on both microscopic bulk properties (atomic/molecular absorption) but also on surface structure- textured surfaces can appear black under some conditions.
I used to design electrical enclosures and there was a white paint that had an IR emissivity above 0.9 but my boss always thought I was crazy since "white doesn't radiate".
jobyts
#6
Jan24-08, 02:25 PM
P: 216
Quote Quote by DaveC426913 View Post
Without complicating the matter too much:

Black objects are black because almost all the light that falls on them is absorbed into the material. Little or no light is reflected back toward your eye, therefore you see black. That light that is absorbed ultmately becomes heat.

White objects are white because almost all the light that falls on them is reflected by the material. Because all the light is reflected back toward you eye, you see white. Little light is turned into heat.

Red objects absorb light at frequencies other than red and tend to reflect frequencies near red. They don't get as warm as objects that are black.


The nature of an object's colour has to do with the molecules and their electrons. Electrons in different states absorb different frequencies. When we build things - and especially when we paint them - we choose materials that contain molecules with very specific reflection frequencies. For example, Phthalo green - a paint colour - has very stable molecules in it that reflect light in a very specific, predictable band of green. Lamp black - another paint colour, has molecules that absorb ALL frequencies of light.
Is it correct to say, white objects do nothing with the incident EM waves, and black objects change the frequency (to IR range)?
TVP45
#7
Jan24-08, 03:25 PM
P: 1,127
No, it's not an "all or nothing" case. White reflects maybe 60% to 90% of incident light depending on the actual pigment and the shininess of the surface. Black reflects maybe 5% to 30% for the same reasons. Light energy absorbed will be changed to IR and radiated according to the emissivity at that wavelength.
DaleSpam
#8
Jan24-08, 05:07 PM
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What always surprises me when I think about these kinds of things is shiny black objects! How weird. They reflect a lot of light (specular reflection), but they are black (diffuse absorption). Definitely a "surface structure" effect as mentioned above, but still interesting.
DaveC426913
#9
Jan24-08, 05:09 PM
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Quote Quote by jobyts View Post
Is it correct to say, white objects do nothing with the incident EM waves, and black objects change the frequency (to IR range)?
Too simplistic and misleading.

If a bond in a molecule absorbs a given photon of a given frequency, then that photon will be converted to kinetic energy of the bond - the bond will wiggle more. Kinetic energy = heat so the object is slightly warmer.

If the bond is impinged upon by a photon that is not of the right frequency, the bond will not absorb it. Depending on the structure, the photon (actually, the photon wavefront, but that's another story) will either pass straight through the material (meaning the material is transparent), or it will reflect off it (meaning the material is both opaque and reflective to that frequency of light.)
dst
#10
Jan24-08, 05:10 PM
P: 389
Quote Quote by DaleSpam View Post
What always surprises me when I think about these kinds of things is shiny black objects! How weird. They reflect a lot of light (specular reflection), but they are black (diffuse absorption). Definitely a "surface structure" effect as mentioned above, but still interesting.
Is that caused by a transparent & reflective layer on top of the black layer? An example would be anodized aluminium or powdercoating on, well anything. What raw material can actually do both without any change in composition?
jobyts
#11
Jan24-08, 06:04 PM
P: 216
Quote Quote by DaveC426913 View Post
Kinetic energy = heat so the object is slightly warmer.
Why is kinetic energy == heat , for every substance?
jobyts
#12
Jan24-08, 06:11 PM
P: 216
Quote Quote by Andy Resnick View Post
Hang on... just because and object is black (highly absorptive) in the visible just not mean it is also black in the infrared. Snow is black in the IR region, for example.
I was wondering how would any substance can be black for the infrared. If a substance reflects all the infrared rays, it should be white for the IR rays. If it absorbs the IR rays, it would cause the lattice to giggle, and emit heat, which is again in IR. So in any case, it should emit IR, right?
DaveC426913
#13
Jan24-08, 07:18 PM
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Um. I am not sure exactly which frequency of light would cause the lattice to giggle.



DaveC426913
#14
Jan24-08, 07:18 PM
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Quote Quote by jobyts View Post
Why is kinetic energy == heat , for every substance?
That's what heat is.
Andy Resnick
#15
Jan25-08, 09:38 AM
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Quote Quote by jobyts View Post
I was wondering how would any substance can be black for the infrared. If a substance reflects all the infrared rays, it should be white for the IR rays. If it absorbs the IR rays, it would cause the lattice to giggle, and emit heat, which is again in IR. So in any case, it should emit IR, right?
Part of the conceptual problem may be with the concept of 'black'- it's typically associated with visible colors, not with thermal imaging.

Absoprtion = emission, by thermodynamics. A 'black body' is a perfect emitter in addition to being a perfect absorber. If the temperature of a black body is higher than the environment, it will be brighter than the environment. If it is colder, it will appear darker than the environment.
Andy Resnick
#16
Jan25-08, 09:44 AM
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Quote Quote by DaveC426913 View Post
That's what heat is.
I disagree. Kinetic energy is a measure of the state of a system, is associated with how much work can be performed and is an exact differential. Heat, on the other hand, is the difference between the internal energy and the work, is not an exact differential, and is associated with a *process* rather than a state.
meeeee5
#17
Jan25-08, 11:36 AM
P: 2
Quote Quote by DaveC426913 View Post
That's what heat is.
absolutely, heat is the process of energy transfer, and the kinetic energy is equal to 3/2*K*T.
jobyts
#18
Jan25-08, 01:32 PM
P: 216
Let me be clear on some basics. Please correct if I am wrong.
Energy is a measure on the ability to do work on matter. Matter can do work on other matter, which is kinetic energy (eg; a moving object). EM wave can do work on matter, hence they have energy too. Heat is a just IR wave in the EM spectra.

In my understanding, the connection between EM wave to kinetic energy comes as follows:
An EM wave hits an atom; cause it to do work on the atom (causing the electrons to move; EM energy to kinetic energy conversion). Then the electron emits another EM energy which is Infrared ray, which we call heat.

My question is what property of the matter/electron caused it to emit waves in the IR frequency. Why not in some other frequency?


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