The colour of an atom?

Krishna: Atoms do not have colours they are clear except under special conditions. You cannot see the colour of a single atom because it is too faint.

Colours are not actual colours. We see things because they reflect light into our eyes. Colours don’t exist outside of our visual perception. Different animals see the world in different colours depending on the way their eyes developed.

The colour of something is the colour of light that it reflects most when light of all colours shines on it. A green leaf is green because when the sun shines its light on the leaf, the colour that the leaf reflects best to our eyes is the green. The sun's light contains all the colours of the rainbow (red, orange, violet, yellow, green, blue, indigo) but the colour that is reflected most to your eye is the green.

The part of an atom or molecule that "reflects" the light is the electrons on the outside of the atom. Now..the colour comes in two steps (1).

The electrons first absorb some of the light that hits the atom or molecule. The electrons which absorbed the light then emit (give out) some light.

If the electrons give out exactly the same light as they absorb, the substance is "colourless". However, if the colour of the light emitted is different from the colour absorbed, then the substance has a colour. In the case of the leaf, the leaf absorbs all the colours but only reflects green.

For atoms and regular lighting - whatever the atom - the absorbed light and the emitted light are the same. That is because for a single atom the electrons have to absorb and emit the same light.

That is why literally speaking they are ‘colourless’.

But according to some people of science, the answer really depends on how you define "having a colour". The term "colour" refers to visible light with a certain frequency, or a mixture of visible light frequencies. Therefore, the word "colour" describes the frequency content of any type of visible light. Anytime visible light is present, we can describe it as having a certain colour. With this in mind, there are many different ways an object can reflect or emit visible light. Thus, there are many ways an object can "have a colour". While a single, isolated, atom can reflect or emit visible light in several of these ways, it does not participate in all the ways. If you define "having a colour" very narrowly such that it only includes certain mechanisms, then atoms do not have colour. If you define "having a colour" more broadly, then atoms do have a colour. Let us look at the different ways an object can reflect or emit visible light and apply each one to an atom.

1. A single atom is too small to have a colour when you take bulk reflection, refraction, and absorption into account.

2. If we expand the definition of "having a colour" to include thermal radiation, individual atoms still have no colour.

3. However, if you take Rayleigh scattering into account ….

Called "long-wavelength scattering", Rayleigh scattering is when light does bounce off of single atoms and molecules. But because the light is so much bigger than the atoms, Rayleigh scattering is not really the "bouncing" of a light wave off of a small particle such as an atom, but is more a case of immersing the particle in the electric field of the light wave. The electric field induces an oscillating electric dipole in the particle which then radiates. Because the mechanism is so different, Rayleigh scattering of white light off of small particles always creates the same broad range of colours, with blue and violet being the strongest. The colour of Rayleigh scattering is always the same (assuming the incident light is white) and is mostly independent of the material of the scattering object.

Therefore, a single atom does have a colour in the sense that it participates in Rayleigh scattering!

4. Gas discharge (e.g. a Neon light) is perhaps the mechanism that would best fit the notion of an individual atom "having a color". Gas discharge is what happens when you take pure atoms, isolate them from each other in a low-density gas state and then excite them using an electric current. When the atoms de-excite, they emit visible light. The key here is that a particular atom can only being excited, de-excited, and emit light in certain ways. This leads to the color of an atom during gas discharge being very strongly tied to the type of atom involved. The frequency spectrum of an atom during gas discharge is considered the color "fingerprint" of that particular type of atom.

For instance, true neon signs are always red because neon atoms themselves are red under gas discharge. Argon atoms are lavender under gas discharge, while sodium atoms are yellow and mercury atoms are blue. Many of the colors generated by "Neon" lights are attained by mixing different gases together. The "flame test" used in chemistry to detect certain atoms is essentially a less-controlled, less-pure version of a gas discharge lamp.

As opposed to gas discharge, which forces an atom to emit all of its characteristic colors; florescence, phosphorescence, and laser emission all involve exploiting certain transitions so that only certain atomic colors are emitted. They can be considered special cases of gas discharge, as far as atomic color characterization is concerned.

There are many other ways an object or material can emit or reflect visible light; such as through semiconductor electron-hole recombination (in LED's), Cherenkov radiation, chemical reactions, synchrotron radiation, or sonoluminescence; but all of these involve the interaction of many atoms or no atoms at all, and so are not pertinent to the current discussion.

In summary: in the sense of traditional reflection, refraction, absorption, and thermal radiation, individual atoms are invisible. In the sense of Rayleigh scattering and gas discharge atoms do have a colour.

Footnotes:

1. Fermilab
2. Does an atom have a colour?