simple amcas magnification question. HELP

[Neplina94]

In the above figure, an object O is at a distance of three focal lengths from the center of a convex lens. What is the ratio of the height of the image to the height of the object?

My question is simple:

I get why the answer is 1/2. But why is it not negative given that m= -i/o ??????

please help

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[AdaptPrep]

Hi, neppywise--

If they were asking for the magnification, you are correct, it would be -1/2 based on the formula you used. However, they asked "what is the ratio of height of the image to the height of the object". They are not asking about the orientation of the image (the other part of magnification). The formula to use for ratios is h'/h = s'/s (where h' is image height, h is object height, s' is image distance, and s is object distance--all are positive in this case). It is analogous to: speed is the magnitude of velocity but not the direction. They simply wanted the ratio. I hope that is how to explain it (otherwise I am confused as well).

 

[Neplina94]

Hi, neppywise--

If they were asking for the magnification, you are correct, it would be -1/2 based on the formula you used. However, they asked "what is the ratio of height of the image to the height of the object". They are not asking about the orientation of the image (the other part of magnification). The formula to use for ratios is h'/h = s'/s (where h' is image height, h is object height, s' is image distance, and s is object distance--all are positive in this case). It is analogous to: speed is the magnitude of velocity but not the direction. They simply wanted the ratio. I hope that is how to explain it (otherwise I am confused as well).

perfect explanation. Thank you!