(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A slide projector needs to create a 81 cm high image of a 2.0 cm tall slide. The screen is 360 cm from the slide. Assume that it is a thin lens. What focal length does the lens need? How far should you place the lens from the slide?

2. Relevant equations

1/f = 1/s + 1/s' (f is the focal length, s is the object distance, s' is the image distance)

M= h'/h = -s'/s (M= magnification)

3. The attempt at a solution

This problem is driving me crazy! From the wording of the question, it sounds like s' + s= 360 cm. I also know that M= h'/h= 40.5cm, and therefore -s'/s = 40.5cm. Almost everything I've tried has involved these equations.

I tried plugging 360-s' in for s, so -s'/(360-s')=40.5. But when I solved this I got 369.1 cm for the image distance, and I don't see how that can be right given that this is more than the image distance and the object distance added together. It should be less than 360 cm.

All the other substitutions I've tried have gotten me nowhere, as I keep running into the same wall- even if I get an equation down to only s' or s, I still know neither value and can't solve it. :(

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# Homework Help: HELP! Finding the focal length of a lens, I'm totally stuck

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