I'm not sure whether to use a lens with a -15 focal length, but is that even possible?
I think it's supposed to be about converging and diverging lenses; does one have a positive focal length and the other a negative one?
If you had to straighten out a refracted ray that went through a lens with a focal length of +15 cm, what could you do?
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Yes, a +15 focal length followed or preceded by a CLOSE -15 focal length lens would result in a "straightened out" beam, and no image would be formed.
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