1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Photons emitted per sec.

  1. Mar 14, 2010 #1
    1. The problem statement, all variables and given/known data

    An LED emits a light beam with a power of 10 mW. The wavelength of the light is 420 nm. How many 420nm photons are emitted per sec.?

    2. Relevant equations

    E=hc/lamda is 2.95E-9

    P/E= Photons emitted per sec.


    3. The attempt at a solution

    2.95E-11 is wrong.

    2.1E16 photons per/sec. is the answer
  2. jcsd
  3. Mar 14, 2010 #2


    User Avatar
    Homework Helper

    Your value for E is wrong.

    h=6.626068 × 10-34 m2kg/s
    Last edited: Mar 14, 2010
  4. Mar 14, 2010 #3

    (6.626x10E-34)(3.0x10E8)=1.9878E-25 / 420nm = 4.7328E-28

    (10x10E-3) / (4.7328E-28) = 2.1129E25

    This is still not 2.1E16
  5. Mar 14, 2010 #4


    User Avatar
    Homework Helper

    You forgot to convert nm to m.
  6. Mar 14, 2010 #5


    User Avatar
    Homework Helper

    Check over your exponents in the part in red, it is not E-28.
  7. Mar 14, 2010 #6
    1.9878E-25/4.2E-7 = 4.7328E-19

    (10x10E-3) / (4.7328E-19) = 2.1E16

    Thank You!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook