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A soap bubble has index of refraction of 1.33. What minimum thickness of this bubble will ensure max reflectance of normally incident 530 nm light?

Ans is 99.6, but how do I get that?

I am thinking I could use the formula $\frac{2nt}{\lambda}$ = some integer. And since its a MCQ I can test all values (24.9, 76.9, 99.6, 199, 398 - all nanometers). And I can see 99.6 will give 4.999 which is reasonably close to an integer. 199 and 398 also give close to integer values, but I suppose I omit them since question asks for min thickness? Or how should this problem be done? Eg. if its not a MCQ?

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