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Homework Help: Explain this

  1. Oct 22, 2005 #1
    I have the answer to this question but I am not exactly sure how I got it. Can you explain this to me and show me how to work it.

    a. When ultraviolet photons from the sun with a wavelength of 292 nm shine onto our skin they provide enough energy to break bonds in our DNA; this can result in skin cancer. Calculate the energy of this photon in Joules.
    b. Why can't green light (510 nm) or red light (630 nm) give us skin cancer the same way ultraviolet does?
    My answer : 6.858 x 10^-28
    I do not even know if this is correct, it has been a while since this was done. I am studying and cannot remember it. Thanks.
  2. jcsd
  3. Oct 22, 2005 #2
    [tex]E = h \times v[/tex]

    Gives the energy of a photon E by multiplying its frequency v by plancks constant h ([tex]6.63 \times 10^{-34})[/tex]).

    [tex]v = \frac{c}{\lambda}[/tex]

    Gives the frequency v of the photon by dividing the speed of a photon (the speed of light) c (about [tex]3 \times 10^8[/tex]) by its wavelength [tex]\lambda[/tex].

    A bigger wavelength means a smaller frequency because the wavelength is inversely proportional to the frequency, and a smaller frequency means a smaller energy because the energy of a photon and its frequency are proportional.

    Therefore green and red light which have bigger wavelengths will have smaller frequencies and so their photons will have less energy to break the bonds in our DNA and cause skin cancer.
    Last edited: Oct 22, 2005
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