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Author Topic: What causes color?  (Read 4081 times)

Offline tdclemens

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What causes color?
« on: 24/01/2010 04:47:07 »
suppose we have 10 grams of absolutely pure gold. That gold appears yellowish when white light shines on it.

What causes this element gold (which has a specific atomic chemical structure) to give off this yellowish color? We all know that color is determined by its wavelength within the visible spectrum. White light consists of a lot of different wavelength photons.

My guess is that some of these photons are absorbed, and others reflected. But if some photons are absorbed, where does this energy go?

any thoughts? ...



 

Offline JP

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What causes color?
« Reply #1 on: 24/01/2010 08:05:08 »
That's a really insightful question.  It sounds like you already know that the yellow light you see is caused by certain wavelengths of visible light being selectively reflected.  The rest of the visible light is presumably absorbed, which means its energy has to go somewhere.  Most likely that energy goes into vibrations, rotations, or other kinds of motion of the molecules making up the object.  In the case of a metal like gold it could also go into the electrons which can move around somewhat freely an create small currents in the metal.  This energy probably ends up mostly as heat.  If you leave a yellow object in the sun, it will definitely warm up from absorbing some of the light.   It's also probable that some of this energy gets transferred into electromagnetic radiation leaving the object, either visible light or otherwise, but the dominant visible light leaving the object should still be yellow.
 

Offline tdclemens

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What causes color?
« Reply #2 on: 25/01/2010 05:21:52 »
Thanks for the insightful answer! Anyone know why metals get more hot in the sun than other materials?
 

Offline LeeE

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What causes color?
« Reply #3 on: 25/01/2010 12:20:29 »
Thanks for the insightful answer! Anyone know why metals get more hot in the sun than other materials?

I think there are two aspects to that question.  First of all, the amount of energy absorbed by a material, and how hot it gets as a result, depends more upon how reflective it is, rather than what it's made out of.  For example, I was able to compare two sets of exterior metal handrails that were both in sunlight and the only significant difference between them was that one was painted white and the other was unpainted.  The unpainted handrail was much hotter to the touch than the white painted one.

Secondly, the type, form and quantity of material will affect how hot it feels.  For example, you can take a flattened sheet of aluminium cooking foil straight from a heated oven and handle it without burning your fingers because there's so little material in the foil where you're touching it that it can't store a significant amount of heat.  Compact or crush the sheet into a ball though and you will risk burning yourself.
 

Offline lightarrow

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What causes color?
« Reply #4 on: 25/01/2010 14:44:45 »
Thanks for the insightful answer! Anyone know why metals get more hot in the sun than other materials?
Have you tried to put under the sun a perfectly smooth silver surface and a black powder? Which gets more hot?

Instead, if you mean to make an experiment with different materials but which have the same surface absorbance and specific heat, then you have to invoke thermal conductivity: a metal is usually much more conductive than other materials, so it propagates heat inside the material more than an insulator, becoming more hot.
« Last Edit: 25/01/2010 14:53:44 by lightarrow »
 

Offline yor_on

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What causes color?
« Reply #5 on: 26/01/2010 16:44:52 »
Yep Lightarrow, nice pointer that one. Sometimes one forgets :)
As for why we see certain colors and not others?
That's a really interesting question to me.

It seems as some prehistoric fishes saw colors too?
There are some fish that is supposedly very ancient still living in Australia that have a similar set of cones and rods as we humans?

And then you have animals able to see in the infrared spectrum, snakes f.ex.
Take a look at and become flabbergasted
 

Offline lightarrow

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What causes color?
« Reply #6 on: 27/01/2010 01:11:04 »
Yep Lightarrow, nice pointer that one. Sometimes one forgets :)
As for why we see certain colors and not others?
That's a really interesting question to me.
Do you mean in a metal surface? They say that the yellow colour of gold, for example, is due to unusual energy levels of relativistic-moving electrons...

Quote
It seems as some prehistoric fishes saw colors too?
There are some fish that is supposedly very ancient still living in Australia that have a similar set of cones and rods as we humans?

And then you have animals able to see in the infrared spectrum, snakes f.ex.
Take a look at and become flabbergasted
You will start a new thread, maybe...
 

Offline yor_on

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What causes color?
« Reply #7 on: 27/01/2010 01:43:49 »
hmm, you might be right there :) New thread I mean.
But colors do interest me. And the way we animals seem to have sorted it out due to different specializations through our evolution. As for the 'pointer' I meant your comparison with black powder.

But yeah, you're right about gold. it seems a relativistic material.
Although i don't know if it's unusual?

I find almost everything 'unusual' when one start to look at it :)
Golds color
 

Offline Geezer

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What causes color?
« Reply #8 on: 27/01/2010 06:57:58 »
Thanks for the insightful answer! Anyone know why metals get more hot in the sun than other materials?
Have you tried to put under the sun a perfectly smooth silver surface and a black powder? Which gets more hot?

Instead, if you mean to make an experiment with different materials but which have the same surface absorbance and specific heat, then you have to invoke thermal conductivity: a metal is usually much more conductive than other materials, so it propagates heat inside the material more than an insulator, becoming more hot.

The surface treatment could be very important. Let's try this:

Take two identical blocks of gold (if you can't afford gold, copper might be a good substitute).

Paint one with matt black paint. Have the other one chromium plated.

Allow both blocks to heat in the Sun for 45 minutes.

Plunge both blocks into beakers containing one liter of water.

Measure the temperature of the water in each beaker.

Are the temperatures different? If they are, why are they different?



 

Offline mystyle

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What causes color?
« Reply #9 on: 27/01/2010 07:14:50 »
It is obvious that metal is a good conductor of heat and UV rays from sun carry heat which strike in surface of metal cause heat to flow.
 

Offline Geezer

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What causes color?
« Reply #10 on: 27/01/2010 07:29:18 »
It is obvious that metal is a good conductor of heat and UV rays from sun carry heat which strike in surface of metal cause heat to flow.

Obvious is cool. We like obvious. Er, why is it obvious, exactly?
 

Offline yor_on

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What causes color?
« Reply #11 on: 28/01/2010 12:39:24 »
Yeah mystyle?

Explain :)
 

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What causes color?
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