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What does colour depend on ?

  1. Jul 14, 2009 #1
    what does colour depend on frequency or wavelength?
  2. jcsd
  3. Jul 14, 2009 #2


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    Those are two sides of the same coin. For all EM radiation, including light, frequency and wavelength are related by c. To convert one to the other divide c by the value to be converted. Example:

    The two-meter ham band centers around 144 MHz. The wavelength of this frequency is 300,000,000 ms^-1 / 144,000,000 MHz = 2.08 m.
    Last edited: Jul 14, 2009
  4. Jul 14, 2009 #3
    im asking a basic question for optics . at a high school level k? wat m sayin is that if light depends on wavelenght so then when underwater y doesnt all colour outside appear different as acc to snells law light should change wave lenght .???
  5. Jul 14, 2009 #4


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    Because it also changes velocity by exactly the same proportion. The two are inextricably linked.
  6. Jul 14, 2009 #5


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    Please, reasonably legible grammar and spelling are part of the Forum Rules, which you agreed to when registering.
  7. Jul 14, 2009 #6
    That is a really good question that most people ask, the simple answer is that the frequency does not change. This is because the velocity and and wave length change by the same percentage. I would say that the colour of light depends on the frequency, though when c is a constant you can pretty much say that the only other thing that changes the frequency is the wave length. If the frequecny was to change, there would have to be some kind of mechanism for the energy to change at the boundary, in which there is none.

    The index of refraction comes from [tex]\frac{\lambda}{\lambda_{n}}[/tex] where [tex]\lambda_{n}[/tex] is the wave length in the material and [tex]\lambda[/tex] is the wave length in a vacuum.
  8. Jul 14, 2009 #7
    Color is not frequency, by the way. Color is a pecieved value, and is relative to the array of colors in which it is embedded.
  9. Jul 15, 2009 #8
    Yeah, I think it was wrong for me to say that the colour of light depends on frequency. Though that is what it looks like on those scales for electro-magnetic waves. And it is true that colours can look different depending on the shades of colours around them.
  10. Jul 15, 2009 #9
    Its true that perceiving colours depends on the shade of colours around them. Also true is the fact that the colour also depends on the illumination of a room. Assuming perfectly white light and a perfectly white background, the colour we perceive does in fact depend on frequency and not wavelength. In any other medium, given the same illumination and background, the colour remains the same thus adding emphasis to the previous statement.
  11. Jul 15, 2009 #10


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    Color are typically the perceived color of a combination of frequencies (and intentisies). Some colors cannot be duplicated with a single frequency. Most humans have 3 color receptors (some have 4), each which have bell curve shaped sensitivities to frequencies, centered about different frequencies, but these frequencies aren't exactly red, green, blue (or what ever the 4th receptor is with some humans).

    There's always wiki

  12. Jul 15, 2009 #11
    Yes, there are cone cells and rod cells. One of them is for determining colour and the other for general brightness. Google will help you if you are interested. What i meant by colour in my previous post was determining the type of EM wave (red, yellow, uv, etc). That depends only on the frequency. Also note that energy depends on the frequency and not the wavelength as both wavelength and velocity change in different mediums.
  13. Jul 15, 2009 #12
    "Please, reasonably legible grammar and spelling are part of the Forum Rules, which you agreed to when registering."

    I dont see anyone having a problem reading mate.???
  14. Jul 15, 2009 #13


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    Huh? how could we explain redshift if it didn't!
  15. Jul 15, 2009 #14


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    Well, apparently you do since you did not understand what DaveC426913 said. Or is it just that you do not understand what "Forum Rules" mean. Yes, it is possible for people to puzzle out what you really mean. But a lot of people are not going to take that trouble so you are reducing the number of people who might answer you. And some people are just going to think that anyone that bad at English grammer aren't going to understand the answer anyway.
  16. Jul 15, 2009 #15
    The wavelength of light depends on the medium through which it propagates.
    l=c/f where c is the speed and f is frequency. c depends on the medium.

    But this is irrelevant for vision. The light reaching the retina has a wavelength that depends on the medium inside the eye. So no matter if you are in water, air, oil etc, light with the same frequency will have the same wavelength when it reaches the retina.
  17. Jul 15, 2009 #16
    Excellent point nasu. :approve: So the only way to find out whether it depends on wavelength or frequency will be to put the retina in a different medium and check.
  18. Jul 16, 2009 #17


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    I had problems. And I tend to not answer questions that are difficult to read.
  19. Jul 16, 2009 #18
    I agree that wave length and c are change depend on the medium. Wave length will be shorter in a denser medium such as water than in the air, but frequency will be same both in water and in air.

    Thus, if color depend on wave length, then a color that we see in air and in water will be different. That color will be bluish in water, because wave length become shorter.

    But if color depend on frequency, than that color will be the same. Because frequency doesn’t change.

    The fact is we see different color. So the conclusion is color depend on wave length.
  20. Jul 16, 2009 #19
    Well, I know for one, I do not see a different colour when light travels through water. If I did, I would wonder where the engergy was going. Since light waves energy is proportional to frequency.
  21. Jul 16, 2009 #20
    good pt nasu..never occured to me..that it all boils down to the humour in the eye..xD
  22. Jul 16, 2009 #21
    hmmm... nice humour! OK thanks people my mind's got more confused due to difference in opinions . but i'd say i did get some of the stuff . and ya nasu made a good point thanks for that.
  23. Jul 16, 2009 #22


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    You have come to the wrong conclusion.

    If your conclusion were correct, I should be able to stick my head under the water in my bathtub and see a quite demonstrable change in the colours of my bathroom. I should see warm things visibly brighter because I could see the infra-red light emanating from them. Likewise, purples would look black, yet give me a sunburn.

    It is true that large amounts of water will tend to make things look blue. This is not because the wavelngth or frequency changes, it is because reds are preferentially absorbed.
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