colour help

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Offline decepticon

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colour help
« on: 14/12/2007 19:08:55 »
hello everyone, great forum with lots of very clever people in attendance, im starting to get interested in physics and related topics and have been reading about atoms and what they are made from.
Though it is very difficult to understand i realise that they are protons and neutrons in the core and electrons spinning wildly around everywhere but no where! but what is it that gives everything colour? surely atoms must have a colour.


Offline Soul Surfer

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colour help
« Reply #1 on: 15/12/2007 09:16:48 »
Colour is associated with the wavelengths of light that materials reflect.  However this is a property of the bulk material and not the individual atoms which are themselves very much smaller than the wavelengths of light that we see with our eyes.  Individual atoms do not reflect light like bulk materials.  So in that sense individual atoms do not have a colour. 

However that is not the whole story.  When individual atoms have their electron shells disturbed by electricity or heat,  the electrons in the atom can emit and absorb visible light (as well as invisible infra red and ultra violet) for example we are all familiar with the yellow of sodium in street lighting that appears when electricity is passed through sodium vapour.  Chemists and people who enjoy fireworks are also familiar with other colours emitted by various elements when their salts are flame tested.  This emission and absorption is known as the spectrum of an individual atom and is vital for analysing the compositons of stars.
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colour help
« Reply #2 on: 15/12/2007 11:32:12 »
The characteristic emission and absorption lines of elements are only present when the element is is gaseous form. This is the simplest arrangement of the energy levels for the outer electrons on the atom. As soon as the elements are bonded into a condensed state, there are no longer discrete levels. The 'shells' of adjacent atoms have different energy levels because of their proximity and the consequent interactions, so the transitions are no longer what you would expect in a gas - spectral bands, not lines result. A hot solid piece of sodium metal will radiate a continuous spectrum, the same as a hot piece of copper. There will be no characteristic yellow or green colour.
The colours which you see in solid pigments and dyes are due to selective absorption by electrons which are at certain locations within the molecules (the word 'orbit' is not really appropriate) and these will be wideband - rather than lines, due to absorption of some bands of visible frequencies and reflection or transmission of others.
The brightest colours we see in nature are not, in fact, produced by pigments, which, by their nature, absorb energy. The best colours - birds' feathers and insect wings - are produced in the same way that thin oil films on water produce vivid colours. The colours here are produced by interference of light in thin films on the outer surface of the feather. This reflects virtually 100% of light at certain wavelengths and absorbs nearly 100% of unwanted wavelengths, resulting in very intense and striking, almost 'unnatural', colours.