A concave lens forms a virtual image .5 times the size of the object. The distance between the object and the image is 7.9cm.(adsbygoogle = window.adsbygoogle || []).push({});

Find the focal length of the lens. Answer in units of cm.

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i suppose what is confusing me is whether or not the object itself is virtual. i dont see how (with a concave lens) a real object could produce a virtual image smaller than itself.

i can construct a series of equations for this problem fairly easily, but im not doing it correctly apparently.

anyway, if i consider the object itself to be virtual, both Id and Od (image distance and object distance would be negative)...

so:

.5 = -(-Id)/(-Od)

Od = -2Id **

-Od - Id = 7.9 cm **

focal length f = ((1/-2Id) - (1/Id))^-1 **

the equations with asteriks were the ones i used... am i setting them up correctly?

any help is appreciated.

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# Homework Help: Quick Question about lenses/refraction

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