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: Determining the number of photons that reach the earth

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    Light is shining perpendicularly on the surface of the earth with an intensity of 910 W/m2. Assuming all the photons in the light have the same wavelength (in vacuum) of 668 nm, determine the number of photons per second per square meter that reach the earth.

    2. Relevant equations
    The power per area is:
    P/A = (# of photons /t /A)*(energy / photon)

    E/photon = h nu = hc / lambda

    photons /t /A = (P/A) * lambda / hc

    3. The attempt at a solution
    photons /t /A = (P/A) * lambda / hc
    photoms /t /A = (910)*(668x10^-9)/(6.63x10^-34)(3x10^-8) = 3.066x10^21
  2. jcsd
  3. Apr 6, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Looks good!

    Just a small typo, c is 3x10^+8. Looks like you used the correct value for the calculation though.
  4. Apr 6, 2009 #3
    The program I enter my homework answers into doesn't let me have that many decimal places in my answer though (we have to write it out long, it doesnt let us put x10^21), so I was assuming I must've done something wrong.. :-/
  5. Apr 6, 2009 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Looks like 3 significant figures are justified here, given the values of intensity and wavelength.

    By the way, if you are entering numbers into a computer, you can usually write them as
    The "e" is pretty standard notation for computer entry, and simply means "times 10 to the ___ power"

    For example:
  6. Apr 6, 2009 #5
    Oh wow, I never knew I could write my answers that way on this program. haha thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook