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Old Oct 23rd 2010, 09:40 PM   #1
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Join Date: Mar 2009
Posts: 129

In magnification, I keep confusing the signs. I don't know if the signs change when comparing converging and diverging lenses (apart from the focal point). I think that:

(a) real image = positive magnification
(b) virtual image = negative magnification
(c) upright/errected = positive magnification
(d) inverted/upside down = negative magnification.

But then when I try to do #12 on page 757 of Harris Benson's University Physics, "The focal length of a diverging lens is -20cm. Locate the object given that the image is (a) virtual, erect and 20% of the size of the object; (b) real, erect, and 150% of the size of the object."

For (a), I see virtual which means negative and erect which means positive so I conclude that the final magnification is negative. By this I mean 0.2 = -q/p which leads me to the correct answer. However for (b), when I attempt to use the same logic, which is that since the image is real/positive and erect/positive, the concluding magnification should be positive as well, so it should be 1.5 = q/p. But the final answer is -6.67cm according to the solution manual I'm looking at and I do get that answer when I make 1.5 = -q/p. I've been breaking my head for several days now with the signs and I still don't get it so I'd greatly appreciate a breakdown of all the situations that change the signs.

Thanks in advance.
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