# Is there any difference in the colour of a 620 nm light and a 621 nm wavelength light?

My book says there isn't.

But, it also says that every colour in the Visible portion of the Electromagnetic Spectrum has a range of wavelengths. My doubt is-

Let's say from wavelength 'A' nm to 'B' nm corresponds to Yellow Light.So my question is

• Do all wavelengths between A to B correspond to Yellow light? So, (A+1) nm, (B-1) nm, along with A nm & B nm are all exactly of the same colour ? Is there any difference even in their shades?
• Does '(B+1)' nm of wavelength straightaway correspond to Orange light ? Or Is the orange colour going to build up as wavelength increases from B ?
• Realistically, All wavelengths should differ in shades considering the red hue increases in every colour as wavelength increases. Then how come we say that only Seven colours form White light?

PS: Please do clarify all the above three doubts.

• With a good enough spectrometer you can measure the difference. Your Human Mark I eyeball may not be able to tell quite as well. – Jon Custer Nov 13 '19 at 15:34
• Your eyeball is even worse than that - it can't actually tell the difference between light at 620 nm and a certain combination of red and green light, with no "yellow" light at all! (Incidentally, this is why color TVs and LCD screens actually work.) – probably_someone Nov 13 '19 at 15:40
• This question is kinda subjective, could you clarify how you're defining difference? a laboratory instrument clearly can tell, but the human eye cnanot – Alex Robinson Nov 13 '19 at 16:10
• @JonCuster "subject cDa29" has a Human Mark II eyeball. – JEB Nov 13 '19 at 17:56
• @JEB - man, where can I get some of those? Maybe even a Mark III model? Given the current state of mine, I could really use an upgrade... – Jon Custer Nov 14 '19 at 14:41

Wikipedia defines colour as follows:

...is the characteristic of human visual perception described through color categories, with names such as red, orange, yellow, green, blue, or purple. This perception of color derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum.

Colour is then defined solely in terms of human visual perception, so if 620 nm or 621 nm correspond to different colours or hues of colours depend on our capacity to tell any difference between those wavelenghts. Also, the visual spectrum of light is said to be divided in seven colours because we can distinguish seven big categories in the visual spectrum, and that can be analyzed as a result of the phenomenon of scattering of white light through a diffraction plate, and it looks as follows (I couldn't add the full red range, but it doesn't really matter):

Realistically, All wavelengths should differ in shades considering the red hue increases in every colour as wavelength increases. Then how come we say that only Seven colours form White light?

The important thing here is that this spectrum is a $$\textbf{continuum}$$, which results in no clearly defined limit or frontier between colours because of the smooth color gradient that exists. Looking at this spectrum the first thing that we can tell is that there are seven big categories that result in a range (continuum) of wavelenghts that we can very easily distinguish: purple, blue, cyan, green, yellow, orange and red. Looking for example at 550 nm and 600 nm in the previous figure, we can say that both wavelenghts correspond to the same "bag" of wavelenghts which we name $$\textbf{green}$$, but if we carefully observe those colours we note that we can actually tell that they are $$\textit{different}$$ hues of green because of the shading.

Do all wavelengths between A to B correspond to Yellow light? So, (A+1) nm, (B-1) nm, along with A nm & B nm are all exactly of the same colour ? Is there any difference even in their shades?

If A and B define the yellow range, then all colours between A and B are said to be a hue of yellow. Take the wavelenghts that approximately corresponds to A nm, and go 1 nm away from it. Both wavelenghts are certainly inside the "yellow bag" due to the range we define. As this is a continuum spectrum that gradually changes, one can argue that A nm and A+1 nm are in fact $$\textit{different}$$ in the physical sense that they actually correspond to different wavelenghts, and that is in fact the property that characterizes electromagnetic radiation (besides intensity, which is related to the sensitivity of the eye and it is not of much importance in this discussion), but if I handle you a pair of objects that reflect $$\textit{only}$$ A nm and A+1 nm wavelengths, then it is almost certain you that you will not be able to tell the difference between them. This way, in practical terms relative to human perception, both colours are the same kind or hue of yellow. If you go 1 nm away to the left of A nm so you leave the yellow range, a person could not be able to tell the difference at the same time that other may say that it actually corresponds to the green range, and this is one of the subjectivities that arise in defining the limits in these ranges of colour. This way you see that if a pair of wavelenghts is distinguishable (different) for human eye is not unequivocally defined.

Does '(B+1)' nm of wavelength straightaway correspond to Orange light ? Or Is the orange colour going to build up as wavelength increases from B ?

This is the same case as the limit between green and yellow range previously discussed. When you $$\textit{define}$$ the yellow range to correspond to wavelenghts between A and B nm, something like $$\lambda_{yellow} \in [A,B]$$ (it doesn't really matter if it is a closed or open interval), then if you accept to consider seven different categories in the visual spectrum (the colours usually used) you can say that wavelenghts inmediately after B nm are in the orange range, even though B-0.001 nm and B+0.001 nm are indistinguishable by the human eye.

Whenever you talk about colours in the usual sense (not making precise light measurements or anything like that), you usually don't care about 1 nm difference, and that is one of the advantages in defining these categories qualitatively, so there are no problems like those that arise in the frontiers. In the physics realm however, one might talk about wavelenghts or frequencies instead of colours, so again we don't have to worry about ambiguities.

• For crying out loud. To make a connection between color and wavelength you must specify a monochromatic source. This is not optional. The behavior of the visual system allows you to see a color when the incident light has no amplitude at the corresponding monochromatic wavelength. – dmckee --- ex-moderator kitten Nov 13 '19 at 19:54
• Well, Isn't that the idea behind using a good diffraction grating with a polychromatic source ? To split into several wavelengths as I say ?. For a more detailed analysis one can of course add colour filters into the setup, but I don't see the great problem that you set out by analyzing the visual spectrum in the way I put it. – Pablo Navarrete Nov 13 '19 at 20:46
• The problem is that you can see yellow when the light impinging on your eyes has no contribution in the band of where monochromatic light would be percieved as yellow. The detection response in the cones and the processing done in the visual cortex is complicated. Yellow doesnt mean a particular list of wavelengths, it means a response in your visual system. You can make a connection between wavelength and color if--and only if--you restrict your consideration to narrow band sources. – dmckee --- ex-moderator kitten Nov 13 '19 at 20:54

First of all, color is not a property of electromagnetic waves. Color is mainly an illusion created in the brain. The rods and cones in our eye react differently to different wavelengths. We perceive this as a change in its color or hue.

Hence each wavelength corresponds to a unique color. For example blue lies between 450nm and 485nm but the former is darker and the latter is lighter than normal blue.

No one will ever say that 7 colors exist in white light. White light is the combination of ALL wavelengths in the visible spectrum at equal intensity.

• First you state (correctly) that color is the result of how visual signals are processed. Then you add "Hence each wavelength corresponds to a unique color." which really isn't the right way to express the idea. The color response can be linked to wavelength for simple monochromatic illumination, but (a) polychromatic illumination is complicated and (b) even for monochormatic light the detection and processing has rather crude wavelength discrimination. – dmckee --- ex-moderator kitten Nov 13 '19 at 16:07
• Re, "no one will ever say that 7 colors exist in white light." Issac Newton is the guy who first said that a prism splits white light into seven distinct colors. People still say it today. They aren't correct, but they still say it. – Solomon Slow Nov 13 '19 at 16:37
• Re, "White light is the combination of ALL wavelengths." Like you said, color is an illusion created in the brain. You can easily create a color that people perceive as white without using a continuous spectrum. Computer monitors and TV screens for example do it with just three narrow bands of wavelength. – Solomon Slow Nov 13 '19 at 16:39
• I meant to say that no modern physics text book will include such a statement. Also, when an object in real life is white, it reflects each wavelength equally and hence appears to be white. In a way you can say that the white on a tv screen is not true white. – Sam Nov 14 '19 at 3:50
• Good point. The definition of a white surface is very different from the definition of a white light source. A white surface must reflect all visible wavelengths in the same amount. – Solomon Slow Nov 14 '19 at 15:14