F.3 Physics

2007-06-30 9:26 pm

find the position, size and nature of the image formed when an object of height 2cm is place 10cm from a concave lens of focal length 10cm.

besides using graphical method, is there any other methods ?

更新1:

can we use " 1/f = 1/v + 1/u " this formula on calculating concave lens ?

回答 (1)

tweety

2007-06-30 9:39 pm

✔ 最佳答案

by 1/f = 1/u+1/v
1/-10 = 1/10 +1/v
1/v =1/-5
v = -5 (as the value is negative, the image formed is virtual.)

you are reminded that we need to consider positive or negative sign in the formula.
for object distance(u), positive for real object and negative for virtual one. similar for image distance (v).
for focal length (f), positive for convex lens and negative for concave.

2007-06-30 13:43:27 補充：
for the height, we may consider that as linear magnification.magnification = h'/h = h'/2also magnification = v/u = 5/10 = 0.5so h' = 1 cmin calculating magnification, magnitude is concerned only.

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