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A lens forms a real image of $3\ \text{cm}$ high of an object $1\ \text{cm}$ high. If the separation of object and image is $15\ \text{cm}$, find the focal length of the lens.

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Hi Sourav. Welcome to Physics.SE. This is a site which deals with conceptual (+ research-level) Q&A. We don't encourage homework questions that doesn't involve any sort of work done by the author and asks other users to solve the problem. If you think you could clarify your question, please add what you've done along with your question. We're ready to help you. Please have a look at our homework policy or our FAQ for more info. After improving the post, flag it for moderator attention in order to reopen it. – Ϛѓăʑɏ βµԂԃϔ Jan 20 at 15:31
Well, Sorry Sourav. I think you didn't notice my comment. *We expect authors to add their work in homeworks. Your question explicitly asks other users here, to solve your problem - which is very much discouraged here. – Ϛѓăʑɏ βµԂԃϔ Jan 20 at 15:35
so basically i have to show my rough work? – srv Jan 20 at 15:37
Yeah.. So that other users will try to show what's wrong in your work. But, they won't solve the problem and provide you the direct answer for sure. BTW, we use an unique TeX markup called MathJax. The markup is very much helpful in understanding equations, etc. Please have a look here for an introductory, or atleast have a look at our FAQ for an overview. For now, have a look at my revision of your post. – Ϛѓăʑɏ βµԂԃϔ Jan 20 at 15:41
okay. here, h'= -3cm h= 1 cm how can i find the focal length when distance b/w object and image is given?? – srv Jan 20 at 15:43
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closed as too localized by Ϛѓăʑɏ βµԂԃϔ, Manishearth♦ Jan 20 at 16:06

This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, see the FAQ.

1 Answer

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I think you have not paid any effort to solve the question but I am giving some hint. If you use the relation ship of image distance and object distance you can get one relationship and then you can use the magnification of lens to get another relationship. From this two you can get the focal length of the lens.

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