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

Why dont we see color red as blue in water?

  1. Jul 10, 2004 #1
    Just a small question: If the wavelength of light changes in a denser medium then why dont we percieve color red as some other color? :eek:

    Also, why does only wavelength changes not the frequency but not the vice versa? What determines this?
  2. jcsd
  3. Jul 10, 2004 #2
    This is one possible explanation of the result that has little to do with the physics of the matter. Our brains actually takes into account all the information about the colors around the one your considering before it decides what color the wavelength should be percieved as. For a similar example, consider inflation. Let's suppose you want to know Jack's standard of living and I told you he earned $100 a week in 1935 (the wavelength). That information by itself wouldn't be too meaningful because the buying power of $100 over an extended period of time varies. But suppose i also told you how much everyone else made (surrounding wavelengths). Then you could account for inflation and roughly make out Jack's standard of living at the time. Similarly, our brain tracks all the wavelengths in the scene and decides what color each should stand for only after accounting for how lighting (and possibly other factors) alter reflected wavelengths. This is why a red brick stays red as we walk from indoors to outdoors.
    Last edited by a moderator: Jul 10, 2004
  4. Jul 10, 2004 #3
    Wavelength has nothing to do with color. It's frequency that matters. So, red is still red in whatever medium you're talking about. The frequency is never changed, only the wavelength.

    The velocity of light is decreased in an optically denser medium, because the light is absorbed by electrons which become excited, and then decreases when the electrons go back to their rest state. The frequency of the light emitted is directly proportional to the energy level difference between the excited state and the rest state of the electron. Since they absorb and emit the same amount of energy, the frequency of the light is not changed. Instead, the wavelength of the light is changed.
  5. Jul 10, 2004 #4
    I'm not saying my response was correct, but there is something i don't understand about your explanation. As light travels in between being emitted by one atom and absorbed by the next it should be travelling the speed of light because it will be moving through a vacuum in that small time interval. During those times the frequency and wavelength of light will be tightly related. If the frequency of the light is unchanged by an interaction with an atom as you claim, then between atoms, the wavelength should also be unchanged. So when the light actually impacts atoms in the eye, it will not appear any different than it was before.
  6. Jul 10, 2004 #5
    kuenmao said:
    The frequency of the light emitted is directly proportional to the energy level difference between the excited state and the rest state of the electron.

    That brings me to ask a deeper :redface: question: Why does it influences the frequency only but not the wavelength or velocity?
  7. Jul 11, 2004 #6
    It follows from the equation E=hf, where h is Planck's constant.(what was the name of this equation again? I forgot. Check it up in any book that mentions quantum mechanics.) E is energy. A certain frequency of light is related to a certain amount of energy given out when a electron becomes excited to a new state, or gives out energy when returning to a rest state. So, the frequency is fixed, because the electron jumps from a certain position, goes to another, and then returns back to the initial position. The energy absorbed and emitted is the same, and the frequency is always the same.

    Think of picking up a rock from the ground and putting it on the table. That's like a electron absorbing energy and getting excited. Then, the rock falls back onto the ground. That's like giving back out the energy and returning to a rest state. The energy absorbed and emitted is the same.

    Wavelength is something we look at from a macroscopic view. Since the light is absorbed and then emitted from the electrons, there is a small time lapse(or delay) which causes the speed of the wave to slow down. By velocity = wavelength * frequency, we can see that since the frequency is unchanged, and the velocity is decreased, the wavelength is shorter. The decrease in wavelength is a consequence of the decrease in velocity, rather than being directly influenced by the action of refraction.
  8. Jul 11, 2004 #7
    As kuenmao mentioned above, light passing through different mediums will vary it's wavelength and speed, but frequency stays the same. And the reason your eyes always percieve the same color is because of the medium in your eyelobes. Cause even though you're in water, the light still has to pass through your eyelobes to be detected, and that medium never changes.
  9. Jul 11, 2004 #8
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook