\(\frac{1}{f_{a}}=(1.5-1)\left(\frac{1}{r_{1}}-\frac{1}{r_{2}}\right)\)
\(\frac{1}{f_{\ell}}=\left(\mu_{g}-1\right)\left(\frac{1}{r_{1}}-\frac{1}{r_{2}}\right)\)
\({\,^\ell }{\mu _{\rm{g}}} = \frac{{{\mu _{\rm{g}}}}}{{{\mu _\ell }}} = \frac{{1.5}}{{1.25}} = \frac{6}{5}\)
\(\frac{1}{f_{\ell}}=\left(\frac{6}{5}-1\right)\left(\frac{1}{r_{1}}-\frac{1}{r_{2}}\right)=\frac{1}{5}\left(\frac{1}{r_{1}}-\frac{1}{r_{2}}\right)\)
\(\frac{1 / \mathrm{f}_{\mathrm{a}}}{1 / \mathrm{f}_{\ell}}=\frac{0.5}{1 / 5}\)
\(\Rightarrow \frac{\mathrm{f}_{\ell}}{\mathrm{f}_{\mathrm{a}}}=0.5 \times 5=2.5\)
\(\mathrm{f}_{\ell}=2.5 \times \mathrm{f}_{\mathrm{a}}\)
[$\mathrm{h}$ ઊંચાઈ અને $r$ ત્રિજ્યા ધરાવતા શંકુની સપાટીનું ક્ષેત્રફળ $\mathrm{r} \text { is } 2 \pi \mathrm{rh}$ નો ઉપયોગ કરો]