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I am doing my revision and there's such a question. However, when I tried to plug the values into the lensmaker formula, I find that one of the R is equal to zero, as it is not curved.

$(n_2 -n_1)(1/r_1 - 1/r_2)$

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A flat surface has a radius of infinity, not zero... –  DJohnM Jun 10 '13 at 1:37
OH! ok thanks! lemme try it –  Timothy Leung Jun 10 '13 at 1:38
@User58220 that should probably be expanded into an answer –  David Z Jun 10 '13 at 3:07
I don't see what more there is to say: a simple misunderstanding corrected... –  DJohnM Jun 10 '13 at 6:55

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