Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about concave mirrors

  1. Mar 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A concave mirror with a radius of curvature
    of 1.4 m is illuminated by a candle located on
    the symmetry axis 3.4 m from the mirror.
    Where is the image of the candle? Answer
    in units of m.

    2. Relevant equations

    3. The attempt at a solution
    I tried plugging in the numbers into the di=do equation but i think thats wrong out of this world.lol
  2. jcsd
  3. Mar 6, 2009 #2
    Why do you feel the need to plug numbers into an equation? Why not do a ray tracing diagram and find out where the image is formed?

    A ray from the bottom of the candle, situated on the axis of symmetry will be reflected back along that axis.
    Then consider ray from the top of the candle parallel to the axis. Where will that ray be reflected?
    Then consider a ray from the top of the candle passing through the focal point of the mirror. Where will that ray be reflected?
    When you've done all that you'll know where the image is formed.Then measure off the answer from the drawing.
  4. Mar 6, 2009 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Using an equation should work, and is in fact how many physics problems are solved. But you do need to use the correct equation.

    do=di is wrong, since they are not necessarily equal to each other. Can you not find the correct equation in your notes or text book? If not, look here:

    A curved mirror acts like a lens, but you need to be careful about what you use for f.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook