Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Light problem

  1. Dec 21, 2007 #1
    Red light travels from air into liquid at an angle of incidence of 390 and an angle of reflection of 170. Calculate the wavelength of the red light in the liquid if its wavelength in air is 750 nm.

    can anyone give me a clue? ,,, please..
  2. jcsd
  3. Dec 21, 2007 #2


    User Avatar
    Gold Member


    Wouldn't a change in wavelength mean a change in colour?
  4. Dec 21, 2007 #3


    User Avatar
    Science Advisor
    Homework Helper

    Snell's law, Snell's law. Deduce an index of refraction from change in angle. Though, gotta admit, 390 is sort of a strange angle. Once you've done that, wavelength in a medium is related to index of refraction.
  5. Dec 22, 2007 #4
    It would change colour if it moved outside that colour range.... though 750nm should really be classed as infra-red.
    You should know you answer is in the right range when it fall in the red spectrum ;) If you get 400nm you're going wrong somewhere ;)

    I think the angles are suppose to be 39° and 17°
  6. Dec 22, 2007 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Why would the color change? The frequency doesn't.
  7. Dec 22, 2007 #6

    Shooting Star

    User Avatar
    Homework Helper

    First line from "Color vision" in Wikipedia:

    Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect or emit.

    Very misleading, I must say.
  8. Dec 23, 2007 #7
    Shooting star, to be fair since the light has to travel through the 'eye bits' to get to the photoreceptors the wavelength will be shifting around as it goes through the different mediums (speed of light will change). Though from my understanding the photoreceptors responds to the length and not the rate it is hit at before it start to figure out what is what.
    But this takes it off topic (though it never was 'red' light).
    But for interpretation from a wavelength to a colour scale it gets messy :D
  9. Dec 23, 2007 #8

    Shooting Star

    User Avatar
    Homework Helper

    The answer to the problem by the OP lies in the fact that the frequency f will not change, but the wavelength will. This is what we have to assume to solve the problem.

    There seems to be a general consensus (the impression I got from the net) that it is the frequency that the eye perceives as the colour, and not the wavelength. Has this been thoroughly explored experimentally in different situations and different media? You are saying the photo receptors respond to wave length. Are you sure about this? But your next part, about the number of hits, is not very relevant because frequency of light does not have anything to do with number of hits. It has more to do with the energies of the photons, which remain constant.

    Some more reading up on how the eye works seems imperative.
  10. Dec 23, 2007 #9
    frequency is cycles per second.... number of hits of that wavelength in a second 'rate it is hit at'

    Has this been thoroughly explored experimentally in different situations and different media?
    No idea, but I would have thought it may be hard to determine. The media in front of the receptors would be the same for the person interpreting it. So the frequency and wavelength would always have the same relationship. Somebody with a greater understanding about the receptors may know. Hence why I used from my understanding. :) Of course that depends on the amplitude. It may just see it as greyscale.

    Some more reading up on how the eye works seems imperative.
    So very true, but not something I wanted to get in to in an Advanced Physics homework topic.

    EDIT to add
    I also feel we would need to jump out of frequency & wavelength and in to photon energies...
    Last edited: Dec 23, 2007
  11. Dec 23, 2007 #10
  12. Dec 28, 2007 #11

    Shooting Star

    User Avatar
    Homework Helper

    Oh, I thought you were referring to number of hits by photons.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook