Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Wavelength question

  1. Oct 7, 2006 #1
    The speed of light is 3.0 x 10^8 m/s, given the equation c=(x)(v) where v has the unit of s^-1 what is the frequency (v) for the wavelength (x) of 740nm light?

    For some reason I'm not getting the right units; maybe someone can find my mistake:

    3.0 x 10^8 m/s = 740nm(v)

    740nm x (1m / 10^9) = 7.4x10^-7 m

    3.0 x 10^8m/s = 7.4 x 10^-7 m /7.4 x 10^-7

    (3.0 x 10^8m/s) / (7.4 x 10^-7m) = V

    I seem to be having a problem with my units. Can you have m/s/m ?
    What exactly does it mean v has the unit of s^-1 ??

    Anyone explain?
  2. jcsd
  3. Oct 7, 2006 #2
    No need to put nm there. You've already substituted the value for the 'nano' part.


    That seems to be right.

    In a sense, the m's cancel, which leaves you with 1/s.

    v is the frequency of light. In simple terms, it tells you how many wave crests (or troughs) pass a point in a second. It's a number divided by time. Since numbers are not physical quantities with units, you are left with 1/s. There's a name for 1/s; it's know as Hertz, abbreviated Hz.
    Last edited: Oct 7, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook