1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

(Question) Monochromatic light on refraction

  1. Dec 11, 2015 #1
    Okay so I've got a question. If we allow a monochromatic light, say the color green to pass from air to water, what color would the refracted light be? This thought came to me because I was reading that light, when passed through an optically denser medium, would travel slower. Since the frequency of light remains constant on refraction, speed should be proportional to wavelength. So wavelength decreases as a consequence. Should the light be green or a different color?
    Last edited: Dec 11, 2015
  2. jcsd
  3. Dec 11, 2015 #2

    Ken G

    User Avatar
    Gold Member

    Are you asking if our eyes detect wavelength or frequency? Then the answer is, it doesn't matter, because the medium is the fluid in our eye, so has a definite connection between the two. But when we interpret that detection, we should interpret it as a frequency, because that is what stays the same when light passes from the outside medium into our eyes (and note that light refracts in our eyes as well, so we never actually "see the vacuum wavelength" of light).

    Or are you asking how we define color, independently of how our eyes see, like if we had an instrument that reads wavelength in whatever medium it is put in, and calls that the color? If the latter, my guess is we always translate the outcome of the latter question as though it was the former question, i.e., we ask how our eyes would see that wavelength if it passed from the medium of interest, to the medium of our eyes. It's an interesting point, however, because we can in some sense count other wavelengths, like radio or X-ray, as "colors", but of course our eyes don't see those at all. In that case, I would expect that, by the "color" (including radio or X-ray as types of colors, if you will), we always refer to the instruments we actually use, which operate in air typically, or in vacuum, but not in the medium you are talking about. So that wavelength in that medium always ends up getting translated into whatever wavelength it would be in our instrument-- which means we are always talking about the frequency, not the wavelength, as the thing that stays the same in the instrument and in the observed medium in question.
    Last edited: Dec 11, 2015
  4. Dec 11, 2015 #3


    User Avatar
    Science Advisor

    This is a FAQ in this forum. The point is that relevant for the colour impression is that you are seing with your eye. The sensitivity of the colour receptors in our eye are sensitive to frequency, not wavelength. But even if they were sensitive to frequency, this would not make a difference, as the wavelength of the light within the eye is independent of the medium surrounding the eye but only depends on the frequency.
  5. Dec 11, 2015 #4
    Thank you, that explains well.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook