# Wavelengths of the visible spectrum

Why is data about wavelengths of different colors and the visible spectrum in general so different in different sources? On Wikipedia, the numbers differ by up to $\pm\mathrm{30~nm}$.

• It may all be because there's no distinguishable "line" between for example blue and violet. I imagine this is like saying all compounds with species with electronegativity difference of more than 1.9 are ionic; which is very crude, at times leads to wrong results and confusion. – M.A.R. Jun 18 '15 at 16:24
• Many different light wavelengths can produce the same perception of color. Besides, the color perception is not purely physical process, but rather psychophysical and physiological part can differ from one person to another. – Wildcat Jun 18 '15 at 16:29
• @Brian, I knew someone gonna say something like this. But using that logic I can copy Biology.SE site almost entirely and paste its content here, since ultimately all biological processes are rooted in chemical (and physical) processes. But we don't do so, we distinguish the scientific disciplines by focusing on the most important aspects and abstracting out non-essentials. And in this question the actual chemical way receptors work is non-essential. And besides, electronic excitation is a physical process. – Wildcat Jun 18 '15 at 16:46
• I'm voting to close this question as off-topic because I don't feel that it is about chemistry. There is much-much more physics & biology out there. – Wildcat Jun 18 '15 at 16:53
• There is a lot of distracting comment here. Distracting because there is a simple and not-entirely-unrelated-to-chemistry answer. Physiology and philosophy are irrelevant. The reason there are different definitions is because the colour scale is continuous and any division into simple colours is arbitrary and different people's definitions will differ. The specific wavelength of the bright lines in the emission spectrum of sodium, for example, is unambiguous, but whether we classify them as orange or yellow is arbitrary. – matt_black Jun 18 '15 at 23:15

The comments on the question discuss this may not be about chemistry.

One could argue that colors are chemistry related because they are associated with chemical substances: like minerals, chlorophyl, rust, dyes etc.

So colors seem to be properties of substances.

And that's fundamentally wrong.

Color is a function of the visual system of the viewer!
If that sounds surprising, think of color blind people, which see different colors.
Also, there are not just us - there are other animals, with different visual spectrum, partly including what we call UV.

## Colors are not well defined

Actually, there are more than enough ways, even standards, to define which part of the rainbow is green, exactly - people working with visual media like print design can tell details.
And there are way more details than anyone could expect.

So, without a common definition of the colors, it's not the wavelengths that vary in different sources - it's the colors themselves.

To illustrate that color is more complex when looking closely, below you see a diagram defining color names. It is part of a formal standard which is in heavy use: