Published: 3 January, 2007

A single atom, ion or molecule can still emit microscopic light, color

What is the smallest amount of a substance that can still have color?

To answer this question, we need to first know how colors are produced. Color is the human perception of light. Light is characterized by its wavelength (or frequency) and intensity. Visible light covers wavelength from about 380 to 740 nm (nm, the abbreviation for nanometer, is 10^-9 meter). Different wavelength of lights give different colors, for example, green light has a wavelength around 500-565 nm. A substance can appear in color for several different reasons. A green opaque substance appears green to us because it absorbs other colors of light, but not green light, and the scattered green light is seen by our eyes. A red transparent substance is red because it only allow red light to go through it while light with other colors are absorbed. Another way substances can appear in color is fluorescence. A fluorescent substance can absorb light with certain wavelengths (excitation light), and the energy absorbed can then be emitted as light with wavelengths different from those absorbed. If the emitted light is green, then the substance will appear green under the excitation light. Substances can also absorb heat and then release the absorbed energy as light. For example, if you burn table salt using a flame, the salt will emit yellowish color.

With this background information about color, then we can rephrase the question to the following one: what is the smallest amount of a substance that can still absorb light or emit light? The answer to this is a single atom, or ion, or molecule, depending on how the color is generated. For example, burning a single sodium ion would also produce yellowish light just as burning a chunk of table salt. The size of a single atom or ion can be as few as one angstrom (one angstrom is 0.1 nm) in diameter, while a single molecule can be made up of several to millions of atoms.

On the other hand, by rephrasing the question, we assume that our eyes can detect light emitted from any amount of substance, which is certainly not true (I wish I could have that kind of eyes, then I would have vision even better than Superman's). In reality, the amount of light produced from a single atom or a single molecule is too weak to be seen by our eyes. But scientist can use specialized equipment to detect light emitted from a single molecule in some cases. For example, some scientists are using "single-molecule fluorescence microscopy" to study interesting molecules, such as proteins and DNA.