First slide

The lens

Question

The focal length of quartz lens for yellow color is 15.5 cm for which refractive index of quartz is 1.504. Find the focal length of same lens (in cm) for ultraviolet color for which refractive index is 1.434.

Moderate

Solution

$\begin{matrix} \frac{1}{15 .5} = (1 .504 - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}}) \\ \frac{1}{f} = (1 .434 - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}}) \\ \frac{f}{15 .5} = \frac{0 .504}{0 .434} \\ f = 15 .5 \times \frac{252}{217} = 18 cm \end{matrix}$

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