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Question

Why is there no color spectrum at the sub-atomic level? Or is there?
Asked by: Mark 'Socrates' Tusay

Answer

I presume that your question is 'Why don't we ever speak of sub-atomic particles as having 'color'?' To answer the question it is necessary to point out that light is, of course, made up of photons -- each one having its own energy and hence its own frequency. The frequency range of the electromagnetic spectrum that is visible to the human eye (and which we refer to as 'color') is a very small region in the entire range of photon energies we have observed in nature.

That having been said, why do macroscopic objects have color? The sub-atomic particles which constitute everyday objects (protons, neutrons, electrons) are bound together in atoms, molecules, crystals etc. When light falls upon such an object, the energy of the photons may be enough to excite oscillations in these bound structures: it may excite higher atomic energy levels, nuclear energy levels, molecular rotations or vibrations, crystal vibrations, etc. These frequencies of oscillations are (in principle) calculable given the techniques of quantum mechanics and solid state physics.

Once these particles have been excited, they immediately seek to return to the lower energy states from which they came. This is accomplished by the emission of photons. These photons will tend to have the energies of the oscillation frequencies I mentioned above. Naturally, a complicated object will have many, many such possible oscillations frequencies and hence will emit light over a wide range of frequencies. Some substances tend to emit preferably in certain regions of the electromagnetic spectrum -- including frequencies we can see as 'color.' Organic substances are particularly likely to do this (for example chlorophyll tends to emit in the green frequency range -- that's why plants look green).

So now I can answer your question. At the sub-atomic level 'color' is not an intrinsic property of anything. The same electron is just as capable of emitting X-rays, orange light, or radio waves -- all that matters is the environment and interactions it finds itself in contact with. Thus color is rightfully a property of complex substances (molecules, crystals etc.), rather than of the constituents that make them up.
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley


The frequency (color) of light given off in a quantum system is directly proportional to the energy difference between the quantum states between which a transition is made. For example, when an electron in an atom 'drops' from a higher allowed energy state to a lower allowed energy state, light is given off with a frequency determined by:

E = h

where E is the difference in energy between the initial and final states, h is Planck's constant, and is the frequency of the emitted light.

The reason there is no 'color spectrum' as such at the sub-atomic level is that, within the nucleus, the difference between energy levels, delta E, is substantially greater than the differences that typically occur between allowed electron orbits around the nucleus. Therefore, when transitions occur within the nucleus, there is a discrete spectrum of emitted frequencies, but the spectral lines occur at much higher frequencies so that they do not fall within the visible, or color, spectrum that we can see with our eyes.
Answered by: Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA


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