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: Lens equation: pretty simple probably

  1. Jan 8, 2010 #1
    1. The problem statement, all variables and given/known data

    A converging lens ( f = 10.4 cm) is held 8.80 cm in front of a newspaper, the print size of which has a height of 2.04 mm. Find the image distance (in cm) and the height (in mm) of the magnified print.

    D= _____cm
    H= _____cm



    2. Relevant equations

    1/f=1/do+1/di

    3. The attempt at a solution

    1/10.4=1/8.80+1/di

    I know this is the right setup but i feel like an idiot because I forget basic math...I don't remember how to solve this equation so if something can walk me through it step by step that would be great. I only have a simple scientific calculator that I keep using and get a decimal that isn't that right answer so i just don't know
     
  2. jcsd
  3. Jan 8, 2010 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi johnson.3131! :smile:
    Well, there's only two steps …subtract 1/8.80 from 1/10.4, and then 1/x it.

    If you're getting it wrong, perhaps you'd better show us what you've done.

    (you did remember the height is in mm, not cm?)
     
  4. Jan 8, 2010 #3
    this is correct
    1/10.4=1/8.80+1/di
    so
    1/10.4 - 1/8.8 = 1/di

    Your calculator should have a 1/x function to calculate the two fractions on the left and subtract the answers. (The answer will be negative).
    Then take that answer and use 1/x on it to get the value of di. (also negative)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook