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!

Simple laser question

  1. Oct 22, 2007 #1
    Hi I'm a student in school and I'm having a mental block on how to do this question. Could some give me a hint as to the principals I'm meant to consider /use.

    Q) A red, 100mW, laser beam , wavelength 650nm, is incident normally on a perfectly black disk . Calculate the force on the disk.

    I've never really had to relate photons to forces before. I tried consider change in momentum of photon is equal to impulse on black disk but that didn't work.

    Thanks for the help
  2. jcsd
  3. Oct 22, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    You are correct it's just the momentum of the photons ( = h / lambda )
    You can treat the photons just like you were throwing tennis balls at the disc - but remember that the photons are absorbed by the black disc so it is an in-elastic collision.
  4. Oct 22, 2007 #3
    i tried using change of momentum = impulse but I only need to use lambda or Power not both so i think I'm doing something wrong can someone show me a solution.

  5. Oct 22, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    You need both, the power tells you how many photons/sec hit the disc and the wavelength tells you how much effect they have.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Simple laser question
  1. Simple simple question (Replies: 4)

  2. Laser Question (Replies: 2)

  3. Laser question (Replies: 2)

  4. Simple Question (Replies: 3)