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lenses

explorer

2007-09-30 12:52 am

An object of height 5cm is placed at a certain distance from a convex lens .an image of height 3cm isformed on the other side of the lens. the horizontal distance between the object and the image is 30cm.find the focal length of the convex lens.(ans:7cm)

回答 (1)

銘

2007-09-30 1:11 am

✔ 最佳答案

因為
(1/f) = (1/u) + (1/v) f 是focal length u 是object distance v 是image distance

and u + v = 30

u/v = 5 / 3 similar triangle

so, u = (5/3) v
so (5/3)v + v = 30
(8/3)v = 30
v = 90/8 = 45/4
u = 150/8 = 75/4

所以1/f = 4/75 + 4/45
= 24/ 450 + 40 /450
= 64/450
f = 450/64 = 7.03125
所以f = 7 cm~

2007-09-29 17:14:43 補充：
since the image formed on the other side.....the image must be invert.....( convex lens )then,,, there is a similar triangle we can find....and make u/v = 5 /3

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