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Light speed

  1. Dec 25, 2015 #1
    can someone reduce the speed of light?
  2. jcsd
  3. Dec 25, 2015 #2


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    Sure. Put something with a higher index of refraction in its path :smile: .
    But this isn't an answer in the Advanced category ...
    [mentor's note: the thread category was corrected after this post]
    Last edited by a moderator: Dec 25, 2015
  4. Dec 25, 2015 #3
    I thought of it in a different way
    but why? why the does the speed in water is less than it in vaccum?
  5. Dec 25, 2015 #4


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    For electromagnetic waves -- light, for example -- there is a wave equation featuring permeability and permittivity. Speed is inversely proportional to the square root of each of these:$$c={1\over \sqrt{\epsilon\mu}}$$See also Maxwell equations. Physics with a capital P !

  6. Dec 25, 2015 #5
    but I have another question about light
    red things absorb all colors and reflect red
    green absorb all and reflect green ,and so on
    why do they do this?
  7. Dec 25, 2015 #6
    Things don't usually just reflect a single wavelength of light. They reflect many different wavelegths. Your eyes and brain are only capable of percieving a single color at a time, so what you see is a composite of what is actually there.

    For example, if you mix blue paint and yellow paint, you don't see blue and yellow simultaneously, you see green, even though there's no green light there. Your eyes+brain can't tel the difference between green, and blue+yellow.
  8. Dec 26, 2015 #7


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    Such properties are determined by the molecular and their bonding structures of the material. Things that look green must have molecular/lattice structure such that it exhibits resonances at all visible wavelengths except green. In terms of the graphs, green-colored materials will have a dip at wavelength region around 500 nm in the absorption coefficient and a peak in that wavelength in the reflection coefficient. For example take a look at this link http://coolcolors.lbl.gov/LBNL-Pigment-Database/paints/G08.html. There you can find graphs of reflection, transmission, and absorption of some green-colored paint.
  9. Dec 26, 2015 #8


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    This is very true - on a practical level. If a pigment only reflected a very narrow range of wavelengths (= a single wavelength) then it would appear to be very dark / indistinguishable from black. Any useful pigment or colour filter has to present the eye with a nice wide band of wavelengths so as much energy as possible gets to the eye.
    But this is not usually a problem with solids and liquids because the QM involved will not allow narrow responses. EM interacts with solids in 'bands' of frequency.
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