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What determines the color of light -- is it the wavelength of the light or the frequency?

(i.e. If you put light through a medium other than air, in order to keep its color the same, which one would you need to keep constant: the wavelength or the frequency?)

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Thanks for making me think about something I never considered before. –  Arman 2 days ago

7 Answers 7

up vote 16 down vote accepted

For almost all detectors, it is actually the energy of the photon that is the attribute that is detected and the energy is not changed by a refractive medium. So the "color" is unchanged by the medium...

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+1 makes sense, thanks! –  Mehrdad Feb 23 '12 at 0:20

Colour is defined by the eye, and only indirectly from physical properties like wavelength and frequency. Since this interaction happens in a medium of fixed index of refraction (the vitreous humour of your eye), the frequency/wavelength relation inside your eye is fixed.

Outside your eye, the frequency stays constant, and teh wavelength changes according to the medium, so I would say the frequency is what counts more. This explains why object's colour doesn't change when we look at them under (transparent) water (n=1.33) or in the air (n=1.)

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As FrankH said, it's actually energy that determines color. The reason, in summary, is that color is a psychological phenomenon that the brain constructs based on the signals it receives from cone cells on the eye's retina. Those signals, in turn, are generated when photons interact with proteins called photopsins. The proteins have different energy levels corresponding to different configurations, and when a photon interacts with a photopsin, it is the photon's energy that determines what transition between energy levels takes place, and thus the strength of the electrical signal gets sent to the brain.

Side note: I posted a pretty detailed but underappreciated (at least, I thought so) answer to a very similar question on reddit a few days ago. I could edit it in here if you find it useful.

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Wouldn't energy $\implies$ frequency? $E=h\nu$, and $\nu$ is invariant on refraction. Which brings me to an interesting side-question: Materials exhibit colors due to their tendencies to absorb/reflect various wavelengths. What happens when the object is put in a medium? –  Manishearth Feb 23 '12 at 3:14
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Yeah, frequency determines color too. (There is a function mapping frequencies in the visible spectrum to an $\mathbb{R}^1$ subspace of the $\mathbb{R}^3$ RGB color space.) But I emphasized energy because the physical reason that colors are able to be distinguished is really based on the energy. AFAIK the origin of materials' colors is mostly the same mechanism, energy level transitions, so again the colors are unaffected when you put the material in a refractive medium. –  David Z Feb 23 '12 at 5:03

Refraction experiments show it is the frequency that determines color. When a beam of light crosses the boundary between two medium whose refraction index are $(n_1,n_2)$, its speed changes $(v_1=\frac{c}{n_1}; v_2=\frac{c}{n_2})$, its frequency does not change because it is fixed by the emitter, so its wavelength changes: $\lambda_1=\frac{v_1}{f};\lambda_2=\frac{v_2}{f}$. Now, it is an experimental fact that refraction does not affect color, so one can conclude that color is frequency dependant.

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Actually, there is something important all these answers are missing. Color is determined by the response of the human eye, not by energy or frequency. In order to get the full range ('gamut') of colors, I need a mix of red, green and blue light (hence the RGB displays) and the primaries can themselves all be different frequencies. That is, one RGB system can have one frequency for the red, while another has a somewhat different frequency for red, the only hard and fast requirement being that both of them choose that frequency from somewhere in the red range. But the choice affects the gamut.

Now I said "human eye", but of course, other animals see colors, too. Bees see colors into the ultraviolet. But of course, we have no idea what the ultraviolet colors look like to them, only that they do see them, and can distinguish shades of them.

Wikipedia has a lot of good further info on this, but it is scattered among several articles. Probably http://en.wikipedia.org/wiki/Color_theory#Color_abstractions is the best starting point. For something much more thorough and technical, see Poynton's excellent Color FAQ at http://www.poynton.com/ColorFAQ.html

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True and informative, but it remains the energy (i.e. frequency) that determines which photo-receptors are activated. –  dmckee Jul 31 '12 at 13:39

I think it is wavelength. But then wavelength and frequency are related. Longer waves have smaller frequency and vice versa.

As suggested - color is a human (or animal) construct with no specific meaning to light wave (EM radiation)

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Frequency and wavelength are related, but one changes between media (wavelength) and one does not (frequency). Colour is a human experience, but is still rooted in EM, otherwise our perception of colour would be completely arbitrary (but we observe sources giving off similar radiation to have similar colours). –  Kyle Feb 4 at 18:13
    
The energy of the photons is what determines what optical pigments are excited, and energy is coupled to frequency not wavelength. Wavelength is a function of medium. –  dmckee Feb 4 at 18:58

In my opinion both because frequency determines the main category of EM radiation such as: Radio waves, Microwaves, Infrared etc... Inside each category you can access a precise range of wavelenght. So colors are all the combination of frequency in the range 428 THz – 749 THz and wavelenght in the range 700 nm – 400 nm.

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