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B Light: same frequency=different wavelength?!

  1. Mar 20, 2016 #1
    can someone explain why light waves have the same frequency when they pass through different mediums, and at a same time they have different wavelenghts? if frequency=speed of light (which is constant I assume) /wavelength, how could it be? does the speed of light somehow changes when light passes different mediums to get the same result or?
  2. jcsd
  3. Mar 20, 2016 #2
    Yes. It is usually referred to as the Refractive index (click for wikipedia page)
  4. Mar 20, 2016 #3
    thank you!
  5. Mar 21, 2016 #4
    Yes. To share something
    told by my physics teacher, scientists can determine the speed of light relatively "easier" when it moves through a liquid medium like water compared to when it moves through air. (I think its refractive index is close to vacuum? Correct me if I'm wrong)
  6. Mar 21, 2016 #5


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    I'm not sure what was meant by the term "easier" - possibly it referred to the possible accuracy.
    To measure speed, you need to know (in one form or another) a distance and a time. The ratio of speeds in air (space) and water (or many other transparent substances) is only a factor of about 1.5. Your teacher may be implying that the error in measuring the time taken to traverse a given distance would be the same in absolute terms, so the percentage error over a longer time could be less. It's a matter of opinion whether that would be the main source of error.
    Personally, I think it would be 'easier' to do an experiment in air than one involving water (slightly messier to deal with). It's also arguable that any dispersion in the medium could also introduce errors in measuring the edges of pulses.
  7. Mar 21, 2016 #6


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    On the assumption that "easier" means more accurate, our ability to measure time and the speed of light in vacuum are so good that they exceed our ability to measure distance directly. In fact our standard unit of distance is defined in terms of how far light can travel in vacuum in a certain amount of time.

    If it were easier to measure speed in water, they'd have defined distance in terms of how far light can travel in water. But it isn't. And they don't.
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