Wave Number --- How is it defined?

Discussion in 'Physics' started by logearav, Feb 9, 2013.

  1. logearav

    Thread Starter Member

    Aug 19, 2011
    248
    0
    Wavenumber is defined number of waves contained in unit distance. Mathematically wave number is 1/λ and also going by the formula
    c = nλ,
    1/λ is n/c. Going by this formula, n is frequency and c is speed of sound in vacuum that is 3X10^8 m.
    Can we define wavenumber as number of waves produced in a distance of 3X10^8 metre?
    In my exam, i had this question
    Wave number is defined as number of waves
    1)produced in one second
    2)produced in a distance of 3*10^8 metre
    3)produced in a distance of λ metre
    4)produced in a distance of 1 metre.
    I think both 2) and 4) are correct. Am i right?
     
  2. mikeleeson

    New Member

    Aug 22, 2012
    26
    4
    Wavenumber is a measure of SPATIAL frequency: number of cycles in a given length. It is analogous to the common term 'frequency' which is a TEMPORAL frequency: number of cycles in a given time.
    So the answer is given by your first sentence "..number of waves contained in unit distance".
    In the SI system of units, the unit length is 1 metre - so answer 4 is correct.
    Answer 1 gives you the temporal frequency in cycles per second [Hz].
    Answer 2 gives you a number that is proportional to the wavenumber, but is wrong by a factor of 3x10E8 if you are using SI units.
    Answer 3 gives you a value of 1; always.

    I hope this is of some help.
     
    logearav likes this.
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