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Wavelength and linear momentum

  1. Nov 11, 2014 #1
    1. The problem statement, all variables and given/known data
    I had to find wavelenght and linear momenta of fotons with energies of 3eV, 50 KeV and 1.0 MeV

    Are these correct?
    2. Relevant equations
    E=hc/λóλ=hc/E and p= h/ λ

    3. The attempt at a solution
    a. 3eV Hence λ=(6.63E-34*3E8)/3=6.63E-26m
    p = 6.63E-34/6.63E-26 = 1E-8
    b. 50 KeV = 50000 eVλ=(6.63E-34*3E8)/ 50000 =3.978E-30m
    p = 6.63E-34/3.978E-30 = 1.66E-4
    c. 1.0 MeV = 1 000000 eVλ=(6.63E-34*3E8)/ 1 000000 = 1.989E-31m
    p = 6.63E-34/1.989E-31 = 0.0033
    Last edited: Nov 11, 2014
  2. jcsd
  3. Nov 11, 2014 #2


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    Homework Helper
    Gold Member

    Hello mss90,

    Welcome to PF! :)

    Don't forget to first convert the energy (given in units of eV, keV, and MeV) to units of Joules first, before plugging in the numbers.
  4. Nov 12, 2014 #3
    Alright, can you confirm that this is correct:

    3eV * 1.60E-19 = 4.8E-19 J Hence λ=(6.63E-34*3E8)/4.8E-19=4.17E-7m = 0.417µm
    p = 6.63E-34/4.17E-7= 1.59E-27
  5. Nov 12, 2014 #4


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    That looks about right, although there might be some rounding errors going on somewhere.

    By the way, when calculating the photons' momentum magnitude, you can simply use the [itex] p = \frac{E}{c} [/itex] formula (after converting the energy into units of Joules, simply divide that by the speed of light, 3 × 108 m/s, and you have the magnitude of the photon's momentum. That way you don't need to depend on the λ intermediate step as part of the answer).
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