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

Simple qn on light

  1. Oct 27, 2004 #1

    juz wanna noe something...

    does all colour light travel in the same speed in air as in a vacumm?

    If so... then why does a light disperse when it passes through a medium/..

    <note: they disperse due to different speed of light,

    correct me if i am wrong! :biggrin:
  2. jcsd
  3. Oct 28, 2004 #2
    Different colors of light, same as different frequencies of light, or different wavelengths, travel at different speeds through a transparent medium like air. You are correct, the different colors disperse/refract differently. For prisms:
    Blue bends best, red refracts rotten!
  4. Oct 28, 2004 #3
    To expand, the frequency of the light is what determines it's color, and the wavelength is what determines how much it refracts. That is why when 'white light' (a mixture of all colors) comes from the sun and hits earth's atmosphere, blue light, which has the shortest wavelength, refracts all over and spreads out, making the sky blue.

    When you change mediums, the color doesn't change because the frequency doesn't, even though the wavelength does. That is why if you hold a red ball in your hand, then submerge yourself under water, the color is still red, even though the refractive index is higher and the wavelength has changed.

    I'd just like to add that this rule (probably) also applies to non-visible Electromagnetic waves (light you can AND can't see) traveling through any medium. For example, different frequencies of X-rays going through your skin follow the same relationship as do different colors of visible light going through a vacuum or a transparent medium.

    Just a note: It's been a while since I've done this, and I've never really learned it as thoroughly as I should have, so don't trust what I've said 100%, and if someone else here corrects me, they are more than likely right.
  5. Oct 28, 2004 #4
    Ok... thanx
    so... would the wavelength be changed shorter or longer? And how actually does refraction of light affect the wavelength of the ray?
  6. Oct 28, 2004 #5

    Claude Bile

    User Avatar
    Science Advisor

    It depends on whether you are moving from a high to a low refractive index, or a low to high refractive index. From high to low, the wavelength will decrease, from low to high, the wavelength will increase.

    Refraction of light refers to the change in direction of a ray, not the change in wavelength.

  7. Oct 30, 2004 #6

    ok... But then i thought refraction was the change of speed which result in the change of direction of the different colours of rays///...?

    I noe that refraction causes light rays to change ...But y only does refraction in a prism causes light to disperse... but not a normal piece of glass block?

  8. Oct 30, 2004 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It is the angles on the surfaces of a prism that cause the dispersion. If you shape glass or any other transparent media into faceted faces it will create rainbows. Each wavelength of the incident light interacts slightly differently with the media. Thus different wavelengths take different amounts of time to pass though the material.
  9. Oct 31, 2004 #8
    Correct me if i am wrond (which i probably am)


    wavelength = frequency * velocity of light?
  10. Oct 31, 2004 #9
    Velocity=frequency *wavelength

    Therefore when u arrange them, it will be wavelength=Velocity/frequency
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook