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Homework Help: Find wavelength of a quantum of electromagnetic radiation

  1. Mar 26, 2012 #1
    1. The problem statement, all variables and given/known data
    A quantum of electromagnetic radiation has
    an energy of 0.877 keV.
    What is its wavelength? The speed of light
    is 2.99792 × 10
    8 m/s, and Planck’s constant
    is 6.62607 × 10−34J · s.
    Answer in units of nm

    2. Relevant equations
    ... λ=v/(E/h)

    3. The attempt at a solution

    When i solved, i got 1.413728e-9 nm... I have checked my units. can some just help and point me in the right direction
  2. jcsd
  3. Mar 26, 2012 #2
    v is usually written as c when one speaks of the speed of light in vacuum. Also, the double fraction reduces to:
    \frac{c}{\frac{E}{h}} = \frac{h \, c}{E}
    For this answer, you need to know the conversion factor between an electron-volt (eV) and a joule as energy units. Do you know it?
  4. Mar 26, 2012 #3

    Yes i did convert it but i still got it wrong
  5. Mar 26, 2012 #4
    how did you convert it, and what did you get?
  6. Mar 26, 2012 #5
    I did it again and i got 1.414E-8 ... and i think that is in meters. Am i right??

    so that means that the answer is14.14nm

    I used plancks constant in eV's. Its on the ap equation sheet
  7. Mar 26, 2012 #6
    I didn't get that. What did you get for the energy in joules?
  8. Mar 26, 2012 #7
    1.405109518e-16 J
  9. Mar 26, 2012 #8
    This is correct. Now:
    \frac{h \, c}{E} = \frac{6.626 \times 10^{-34} \, \mathrm{J} \cdot \mathrm{s} \times 2.998 \times 10^8 \, \mathrm{m} \cdot \mathrm{s}^{-1}}{1.4051 \times 10^{-16} \, \mathrm{J}}

    The product and ratio of the mantissas, gives:
    \frac{6.626 \times 2.998}{1.4051} = 14.14
    The exponents sum up to [itex]-34 + 8 - (-16) = -10[/itex]. You may read off the units from the above fraction fairly easily.

    What should the answer be in scientific form?
  10. Mar 26, 2012 #9
    so in nm, it would be 1.414
  11. Mar 26, 2012 #10
    yes, except that you need to use as many significant figures, as there are in variable with the least number of significant figures given in the problem. Fundamental constants are usually known to a lot of significant figures.
  12. Mar 26, 2012 #11
    Thank You
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