frequency and wavelength proportions change as you shift mediums so you can't use one to define the other in a general sense - frequency never changes beween mediums while wavelength does.
everything i've ever read about eye physiology & rods & cones use wavelength of light in sensitivity/response curves, but i suppose it could just as well be frequency
At any single given speed wavelength and frequency are inversely proportional, so at any given speed changes in frequency will change color perception.
Frequency and wavelength are defined in terms of the speed of light:
In a vacuum:
ν × λ = c
In general:
frequency × wavelength = speed of light
The speed of light in a vacuum is about 3 × 10
8.
If you're in a different medium then use refractive index n to find the speed of light in that medium:
n = c/v
Since frequency does not change when moving from one medium to another, if speed decreases then wavelength will decrease. I believe that this is essentially the simple explanation for why water and sky looks blue: they are both media that slow down light.
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For the MCAT, I can't imagine having a question asking the test taker to integrate the visual sensation/perception stuff with the optical physics stuff on this level.
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To be honest, I'm not sure that it's as simple as saying "wavelength" or "frequency" is what determines color sensation... regardless of the medium.
Frequency doesn't change from one medium to another, but wavelength does. But also so does energy. The speed of light in a vacuum is constant, but changes in media like glass or water. Energy of a photon is a function of both speed and wavelength:
E = hc/λ
The first question to ask is: What physical mechanism could the eye use to detect wavelength? There is no easy answer to this question.
Alternative question: What physical mechanism could the eye use to detect energy? This is easy:
As I understand it, the physical transduction of light into sensory information is dependent on the energy of the photon, which has to match the amount of energy that it takes to isomerize the visual pigment in the cone cell. This causes a configurational change in the pigment, causing some membrane-bound protein to change conformation which causes a cascade of events that are ultimately perceived as a color.
So while both wavelength and speed change when entering a medium, I think it's really differences in energy that are sensed and perceived as different colors. Energy is something that intuitively makes sense as a physical parameter that would interact with our photopigments. Length (wavelength) does not make as much sense...
As an experiment: Close your eyes and gentle poke them through your eyelids with your fingers. Many people report perceiving colors when they do this. That's because some energy has been added to the system, and it causes changes in cone cell activity. No "wavelength" has been added.
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So, to the OP: color is usually defined in terms of wavelength, and bleargh is right that this changes in different media. However, for the MCAT you should be prepared to convert between frequencies and wavelengths of light given a particular speed of light.
