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Homework Help: Magnification/Focal Length

  1. Mar 17, 2010 #1
    1. The problem statement, all variables and given/known data

    Shaving/makeup mirrors typically have one flat and one concave (magnifying) surface. You find that you can project a magnified image of a lightbulb onto the wall of your bathroom if you hold the mirror 2.0 m from the bulb and 3.7 m from the wall.

    a) What is the magnification of the image?
    b) Is the image erect or inverted?
    c) What is the focal length of the mirror?

    2. Relevant equations

    m = - di/do ???
    1/di +1/di = 1/f

    3. The attempt at a solution
    I'll be honest, I have no clue how to start this problem. Any help would be appreciated
  2. jcsd
  3. Mar 18, 2010 #2
  4. Mar 18, 2010 #3
    Ok thanks! as far as magnification, is the formula i provided correct? and how do i figure out if it is inverted or not?
  5. Mar 18, 2010 #4
    The positive or negative signs in the equations have the following significance

    (from further down in the link I gave you)

    • f is + if the mirror is a concave mirror
    • f is - if the mirror is a convex mirror
    • di is + if the image is a real image and located on the object's side of the mirror.
    • di is - if the image is a virtual image and located behind the mirror.
    • hi is + if the image is an upright image (and therefore, also virtual)
    • hi is - if the image an inverted image (and therefore, also real)

    hi is height of image, ho is height of object

    The formula for these is
    magnification = hi/ho = -di/do

    Just keep a close eye on the negative sign in the equation and be consistent with signs generally, and this will tell you if the image is real and inverted; or virtual and erect.

    The other way of working this out is to draw a ray diagram.
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