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Homework Help: Energy of a light quantum

  1. Jun 2, 2005 #1
    I'm looking for an expression for the "energy of a light-quantum in eV when the wavelength is in nanometers"

    and I'm kind of stumped :confused:

    anyone know this formula?


    thanks
     
  2. jcsd
  3. Jun 2, 2005 #2
    You can use [itex]E = \frac{hc}{\lambda}[/itex], where h is Planck's constant, c is the speed of light and [itex]\lambda[/itex] is the wavelength (in metres). This will give you an answer in Joules, all you need to do then is convert to eV (1 eV = 1.6 x 10^-19 J).
     
  4. Jun 2, 2005 #3
    ok so I can use [itex]E = \frac{hc}{\lambda}[/itex] X 1eV/1.6 x 10^-19 J

    What about the "in nanometers" part?

    The formula will work with a wavelength in nanometers, it will also work with any other size wavelengths..

    not sure what they are asking here.


    thanks
     
  5. Jun 2, 2005 #4

    Andrew Mason

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    The formula applies regardless of the units. You just have to use consistent units. If you use MKS, the energy is measured in Joules (m^2kg/sec^2), c is in m/sec, distance in m and h in Jsec. A nanometer is [itex]10^{-9} metres[/itex].

    AM
     
  6. Jun 5, 2005 #5
    Is MKS just the same as SI units?
     
  7. Jun 5, 2005 #6
    I think MKS is meter-kelvin-seconds which is basically SI units.
     
  8. Jun 5, 2005 #7

    Andrew Mason

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    MKS is metre-kilogram-seconds or SI units (Système International d'Unités) as opposed to CGS (=centimetre-gram-seconds) or FPS (=foot pound seconds).

    AM
     
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