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!

Plane Mirror Problem

  1. Mar 18, 2009 #1
    I'm having problems doing the problem below. The equation in the hint is: I = Ps/(4*PI*r^2) which I have not clue how it relates to this question except for that it's an equation for intensity.

    I know that there will be a image produced a distance d behind the mirror, so there will be another "light source" 3d away from the screen.

    34sqdro.jpg
     
    Last edited: Mar 18, 2009
  2. jcsd
  3. Mar 18, 2009 #2
    We define intensity as

    [tex]I=\frac{P_{power}}{A_{rea}}[/tex]


    If we assume that the source is pointlike, the intensity will spread like a sphere from the source. [tex]4\pi \ r^2[/tex] is the area of a sphere with the radius r. The intensity at the distance r from a pointlike source is thus

    [tex]I=\frac{P}{4\pi \ r^2}[/tex]

    Does that help?

    (I noticed that in your formula you don't have the radius squared. Are you sure that it isn't squared in your book? This is how I would interpert the formula, might be I am wrong)
     
  4. Mar 18, 2009 #3
    oops thanks for pointing out the typo but I still don't understand what power has to do with this problem.
     
  5. Mar 18, 2009 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi DANIELLYMA! :smile:

    It doesn't really matter what power has to do with the problem …

    you're only asked to say how the intensity changes

    use the formula to calculate by what proportion it goes up because of the "extra" source :wink:
     
  6. Mar 18, 2009 #5
    r is the radius of the source, how do I take into account the the 3d distance from the screen?
     
  7. Mar 18, 2009 #6

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    I don't follow :confused: … r is distance.
     
  8. Mar 18, 2009 #7

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Yes, r is "the 3d distance from the screen".
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Plane Mirror Problem
  1. Plane Mirrors (Replies: 4)

  2. Plane Mirror problem (Replies: 1)

Loading...