Wave Number --- How is it defined?

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

  1. logearav

    Thread Starter Member

    Aug 19, 2011
    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
    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.