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The lens equation

  1. Mar 18, 2014 #1
    The lens equation is given as M = h'/h = -s'/s

    It's not explained in the book as to why h' and h are positive and s' is negative while s is positive.

    Could someone explain?
  2. jcsd
  3. Mar 18, 2014 #2
  4. Mar 18, 2014 #3
    I'm taking issue with the book. As the intersection of the light ray where the image formed is inverted, reduced and real, the vertical distance to the horizontal axis is labelled -h'.

    But the given equation is h'/h. For some reason, the -ve sign has vanished.
  5. Mar 18, 2014 #4
    The equation is correct. h'/h is the definition of the Magnification M. If you plug in a positive value for h and a negative value for h' (Inverted image) you get a negative value For M. A negative magnification means you get an upside down image (inverted image).
  6. Mar 18, 2014 #5


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    Staff: Mentor

    First: you need to beware that there are two different sign conventions (rules for assigning + and - signs) that are used in different optics textbooks:

    • Gaussian: Real objects and images have positive distances from the lens. Virtual objects and images have negative distances from the lens.
    • Cartesian: Objects and images on the right side of the lens have positive distances. On the left side of the lens, they have negative distances.

    (In both cases we assume the light travels through the lens from left to right.)

    Some equations have + and - signs placed differently, depending on which convention you're using. The magnification equation you gave above is for the Gaussian convention. However, the diagram you included in a later post shows the Cartesian convention! (as evidenced by the "Note: object distance normally negative.")

    In your diagram, h is a positive number because the object arrow points upward, and h' is a negative number because the image arrow points downward. Therefore M = h'/h is a negative number, which reflects the fact that the image is inverted with respect to the object.

    Using the Gaussian convention, s is a positive number because the object is real, and s' is a positive number because the image is real. Therefore M = -s'/s is a negative number, in agreement with h'/h. The - sign in the formula is necessary in order that both versions of the formula give the same result.

    Using the Cartesian convention, s would be negative, s' would be positive, and the magnification equation would be M = s'/s (without the added - sign).
  7. Mar 18, 2014 #6

    I have managed to derive it.

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