A student investigates the image formed by a converging lens. Refer to Fig. 3.1. The focal length $$\(f\)$$ of the lens can be found using the equation: $$\[ f=\frac{\left(D^{2}-d^{2}\right)}{4 D} \]$$ Use the value of $$\(D\)$$ from (a) and $$\(d\)$$ from (b)(ii) to calculate a value $$\(f_{1}\)$$ for the focal length of the lens. Give your answer to 3 significant figures. $$\[ f_{1}=\ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . \mathrm{cm} \text { [2] } \]$$

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