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# !!!!Magnification!!!?

An overhead projector lens is 32 cm from a slide(which is the object). you want a magnification of -12. what focal length should you use? (Unit=cm)

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- Andrew SmithLv 72 years ago
magnification = - di/do

-> di/do = 12

di = 12 * 32 = 384 cm

Now to find the focal length use 1/f = 1/do + 1/di)

-> f = 1/ ( 1/ do + 1/di) = 1/( 1/32 + 1/384) ~= 29.5 cm

This is one case where you are entitled to claim more significant figures so you might claim that IF ( and only if) the value of 32 meant 32.000 ( ie it meant exactly 32) then you could use more significant figures in the focal length.

- Anonymous2 years ago
24 cm

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