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Homework Help: Help with curved mirrors!

  1. Oct 18, 2009 #1
    1. The problem statement, all variables and given/known data

    "An object is 4.0 cm in front of a concave mirror having a 12.0-cm radius. Locate the image using the mirror equation and a ray diagram."

    2. Relevant equations

    1/f = 1/do + 1/di where f = Focal Length, do = Distance Object, di = Distance Image

    m = hi/ho = -di/do where m = Magnification, hi = Height of Image, ho = Height of Object

    3. The attempt at a solution

    f = 6 cm
    do = 4.0 cm
    Ho = ?
    di = -12
    Hi = ?
    C = 12

    I got 1/6 = 1/di + 1/4
    1/6 - 1/4 = 1/di
    Which equaled -12

    Hi/Ho = -(-12)/4
    Usually I knew Hi or Ho to get the answer but I don't know it this time!
    Can anyone help me with this seemingly easy problem?
  2. jcsd
  3. Oct 18, 2009 #2
    First of all, you found the answer, you just need d this time and show where it is.

    Now then also probably want a diagram there, just to test your understanding of the geometry of the problem.

    Ok, first of all you need to find the image, so you need to draw your ray diagram.

    I wish I could post an image for this, but physicsforums won't let me (you can only link to one), which is a little lame.

    But if you draw a concave mirror, then the focal point, where will be the image compared to it?

    Then draw the image as a perpendicular arrow going up (your book must show this somewhere). Now you just need to draw three rays:
    1. Draw a ray going straight to the mirror from the top of the arrow you drew (perpendicular to the arrow you drew as well). The rule of this ray is that it will bounce to the focal point, so when it hits the mirror, draw a straight line from there to the focal point

    2. Draw a ray that goes from the top of the arrow through the focal point of the mirrror. The rule for this ray is that once it hits the mirror, it bounces back parrallel to the ground.

    3. Draw a ray that goes from the top of the arrow you drew again, but this time going to the center of the mirror. The rule for this one is that you draw a line coming back at the same angle is came in, but downwards

    With these three lines you have, elongate them until they intersect. They may intersect in front of or behind the mirror, so you must elongate them both ways. Where they intersect is the top part of the new arrow you will draw perpendicular to the ground, your image.

    Aah, I'll just draw an example which will be simpler. Use this as a guideline to solve yours.

    And now, for your problem, where is the image located, is it contestant #1, #2, #3?

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