1. Not finding help here? Sign up for a free 30min 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!

Laser Beam Question

  1. Dec 5, 2007 #1
    1. The problem statement, all variables and given/known data

    If a 20-W laser beam, which has an initial diameter of 2.0 mm, spreads out to a diameter of 2.0 m after traveling 10,000 m, what is the final intensity of the beam?

    A. 20 x 10-4 W/m2
    B. 20 x 10-6 W/m2
    C. 20 x 10-8 W/m2
    D. 6.4 W/m2
    E. 6.4 x 10-6 W/m2

    2. Relevant equations

    I found an equation that states if light hits a surface area A at normal incidence, the intensity I is equal to:

    I = <P> / A

    3. The attempt at a solution

    If I draw this beam out, I have a frustrum of a right circular cone.

    The lateral surface area of a frustrum = PI * S * (R + r) where R and r are the radia and s is the diagonal side.

    After a little math, I found s to be about 10000. It is a little over but its neglible.

    So the surface area would be = PI * 10000 * (.001 m + 1.0 m) = 31447.3 m^2

    Wouldnt that mean the intensity I = 20 W / 31447.3 m^2 = .000636 W / m^2

    Intensity = 6.4 * 10^-4 W / m^2

    Obviously, that is not one of the answers. What am I doing wrong here?

    Any help will be greatly appreciated. Thanks in advance.

    Sincerely,
    Travis Walters
     
  2. jcsd
  3. Dec 5, 2007 #2

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    Think of it this way. How much energy (watts) do you have through a 2m diam. circle?
     
  4. Dec 5, 2007 #3
    Hey there,

    For reference, the answer key says 6.4 W/m2 is the correct answer.

    The answer I got was 6.4 * 10^-4 W / m^2, so I am just wondering the answer is a typo.

    I am not sure where you are getting at dlgoff? Please explain :)

    Thanks in advance.

    Sincerely,
    Travis Walters
     
  5. Dec 6, 2007 #4

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    Watts/area. 20W/(3.14xr^2) where r is the radius (1meter).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Laser Beam Question
Loading...