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Problem with units when using E=hv

  1. Jun 11, 2013 #1
    In case I'm wrong, the formula for energy of a photon is E=hf.
    Or E=hc/λ, because f=c/λ.

    Basically the second formula is just in case you don't know frequency so it crams the equation for that into the equation for energy.

    When I use E=hc/λ the units I get are...


    What I did was... A photon of red light, has a wavelength of like 700nM, so...

    (6.626×10-34kg*m2/s2) (3x108m/s) / 7×107m

    = 2.84×1019kg*m2/s3

    I thought the answer would be in Joules, which is kg*m2/s2, so I must be missing something... Why the extra unit of time.
  2. jcsd
  3. Jun 11, 2013 #2
    Wow stupid, planck's constant is J×s which is kg*m2/s

    Well, everything else looks okay?
  4. Jun 12, 2013 #3


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    2017 Award

    Staff: Mentor

    Check the signs of your exponents. Everything else looks right, if you fix your units.
  5. Jun 12, 2013 #4
    Ah right, ^-19

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