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Old Mar 29th 2009, 09:56 PM   #1
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lenses and images

I've been trying to sort this problem out, but I've always had trouble with lenses... I know it's a relatively basic question, but I'm getting a weird answer. I'd really appreciate any help for this question. Thanks in advance!

a)A lens is used to produce a real image of an object such that the image is twice as large as the object. The distance between the image and the object is 90 cm, the focal length of the lens is what?
b) A lens is used to produce an image that is larger than the object. Is the lens converging, diverging, or can it be either converging or diverging?
c) A lens is used to produce an image that is smaller than the object. Is the lens converging, diverging, or can it be either converging or diverging?

Part b and c might be a trick questions (as in, "not enough information to decide"), I'm not sure...
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Old Mar 30th 2009, 12:02 AM   #2
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a)
1/f = 1/ u +1/v where u = distance of lens from object,

v = distance of lens from image,

f = focal length.

f = uv / u+v . Magnification = v/u = 2 in this case. Solve to get f = 20
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Old Mar 30th 2009, 12:05 AM   #3
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True. Is the image real or virtual?
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Old Mar 30th 2009, 05:49 AM   #4
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Thanks for helping with part a) Physicsquest!! Much appreciated.

Part b and c don't mention anything about 'real' or 'virtual' images... b) just says that a lens is used to produce an image that is larger than the object, and c) just says that a lens is used to produce an image that is smaller than the object.
And then you need to say what type of lens it is (converging, diverging), but like I said, it might be a trick question (as in, "it can be either converging or diverging" or "not enough information to decide")

Thanks again for your help
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Old Mar 30th 2009, 10:33 AM   #5
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A diverging lens always makes a diminished image where as a converging lens can make diminished or enlarged image depending upon the position of the object. Hence

For part (b) the lens is converging.
For part (c) the lens may be convergng or diverging.
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Old Mar 30th 2009, 07:24 PM   #6
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Thanks so much for the help Parvez, very much appreciated! I see where I was going wrong for part a), and your explanation for part b and c was really helpful.

Thanks again!!
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