1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Magnification, when is it negative?

  1. Jun 7, 2017 #1
    1. The problem statement, all variables and given/known data
    In magnification, I keep on confusing the signs. From what I understand currently, magnification is positive when the image is erect. An image is only erect when it is a virtual image, therefore virtual images = positive magnification. Vice versa, magnification is negative when the image is inverted, therefore a real image.

    However using the equation m = v/u, m is negative when v is negative. And v is only negative when the image is on the same side of the lens as the object. Therefore meaning a virtual image (According to physicsclassroom "The negative value for image distance indicates that the image is a virtual image located on the object's side of the lens."). This contradicts the statement above where virtual images = positive magnification.

    Where am I going wrong?

    2. Relevant equations
    m = v/u

    3. The attempt at a solution
  2. jcsd
  3. Jun 7, 2017 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    That's only true for a single lens. A multi-element optical system can have positive magnification without the image being a virtual image. Consider a system which uses a relay lens. The objective generates an intermediate image which the relay lens then images, resulting in an upright final image of positive magnification. No virtual image necessary.

    Assuming u is the distance between the lens and the object, then sign convention dictates that it should be negative. If you imagine the lens to be at the origin of a graph, then the object is to the left of 0, which is the negative direction. A virtual image would also be to the left and would also have a negative value for distance. The negatives would cancel out and the magnification would be positive.
  4. Jun 7, 2017 #3


    User Avatar

    Staff: Mentor

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted