If I have 3 LED lights of following colors and wavengths:

Red 650 nm Blue 450 nm Green 550 nm

All three are placed side by side and turned ON.. then what will be the wavelength of the combined light that my eye will see? Most probably the combined color will be 'whitish' because the mixing of RGB colors give white color.

  • 2
    $\begingroup$ They don't add, you just have a mixture of waves with different frequencies. The receptors in your eye relative to wave lengths in the mixture will be excited simultaneously (accroding to the intensity of that wave length). en.wikipedia.org/wiki/Color_vision $\endgroup$
    – Quillo
    Oct 21, 2020 at 12:19
  • $\begingroup$ @Quillo That should be an answer, not a comment. $\endgroup$
    – J. Murray
    Oct 21, 2020 at 13:02
  • 1
    $\begingroup$ What is Gray, from a physics POV? $\endgroup$
    – mmesser314
    Oct 21, 2020 at 13:52

3 Answers 3


Wavelengths are one to one only with the spectral colors, and wavelengths do not add up to a new one, they remain distinct .

It is the perception of color which is a biological function, due to the receptors of the eye, which creates new hews and colors when more than one wavelength falls on the retina of the eye.


The perceived color will depend on the combination of wavelengths according to the plot above.


White light does not have a single wavelength but instead is described by a spectral distribution, which is not uniquely determined. There are many light compositions that can pass as white. The spectrum of the white light from the three LEDs combined is just the sum of the individual LED spectra, weighed with the proper intensity. Likely, your LEDs are also only approximately described by a single wavelength. Other white light may have a different spectral composition but will still look white.

Your eyes sense just 3 colours, red, green and blue. Note that the sensitivity of the cone cells in your retina also has a spectral width about the central color.


Simply, the answer is:

λ_resulting wavelength = (λ_color-1 + λ_color-2 + λ_color-n)/n

So their mixed wavelength would be:

550nm = (650nm + 450nm + 550nm)/3

550nm would be the same wavelength as the green LED and also that of white light which is in the 400 – 700nm range.

White light itself doesn't exist on the electromagnatic spectrum. It's considered "polychromatic" hence white light is reflected into many colors when bounced off objects (colors in the white light which are not reflected are instead absorbed into the material the light bounces off), whereas the LEDs are considered "monochramatic" and therefore their wavelengths are tied directly to the color they emit.


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