1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Geometric Optics

  1. Mar 20, 2013 #1
    1. The problem statement, all variables and given/known data
    An object is placed 400 mm in front of a convex lens of focal length 80 mm. Find the position of the image formed. State the nature of this image.

    A second convex lens of magnifying power X8 is placed 125 mm behind the first convex lens.
    What is the focal length of this lens?
    Find the position of the final image formed and state its nature.

    3. The attempt at a solution

    So the first part is pretty easy,

    [itex]\frac{1}{U} + \frac{1}{V} = \frac{1}{F} [/itex]

    [itex] \frac{1}{V} = \frac{1}{80} - \frac{1}{400} [/itex]

    [itex]V = 100 mm[/itex]

    Next part is where i'm unsure. Here's my attempt,

    [itex]m = \frac{-V}{U}[/itex]

    [itex]8 = \frac{-V}{U}[/itex]

    [itex]8 = \frac{-V}{25}[/itex]

    [itex]V = -200mm[/itex]

    [itex]\frac{1}{U} + \frac{1}{V} = \frac{1}{F} [/itex]

    [itex]\frac{1}{F} = \frac{1}{25} - \frac{1}{200}[/itex]

    [itex]F = 28.6 mm[/itex]

    Is this the correct method?

  2. jcsd
  3. Mar 20, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    The second lens magnifying power is defined (most of the time) by 1/4 diopters + 1. It is not a function of where the object is placed. So 8 = 1/4 d + 1, solve for d, then f = 1/d in meters. But now you have 2 lenses with a distance between them so your job now is to come up with the effective focal length of the two lenses with their separation accounted for.
  4. Mar 20, 2013 #3
    Hmm, that's strange. What if I put this into context and said I was a freshman and had never encountered this formula before?
  5. Mar 20, 2013 #4

    rude man

    User Avatar
    Homework Helper
    Gold Member

    If it's any consolation, I had to look up the meaning of a " ... lens with magnification power of x" also. And I got my degree in 1962!

    And it's always possible the question had a different intent, like maybe the magnification of the object was 8 after inserting the second lens ...
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted