[latex]frac{1}{lambda}=R(frac{1}{2^2}-frac{1}{n^2})\\ frac{1}{lambda}=frac{R}{2^2}-frac{R}{n^2}\\ frac{1}{lambda}-frac{R}{2^2}=-frac{R}{n^2}\\ frac{R}{2^2}-frac{1}{lambda}=frac{R}{n^2}\\ R(frac{R}{2^2}-frac{1}{lambda})=frac{1}{n^2}\\ frac{R^2}{2^2}-frac{R}{lambda}=frac{1}{n^2}\\ frac{1}{n}=sqrt{frac{R^2}{2^2}-frac{R}{lambda}}\\\ oxed{n=dfrac{1}{sqrt{frac{R^2}{2^2}-frac{R}{lambda}}}}[/latex] Pozdrawiam, Adam
[latex]frac{1}{lambda} = R(frac{1}{2^{2}}-frac{1}{n^{2}}) /:R\\frac{1}{lambda R}=frac{1}{2^{2}}-frac{1}{n^{2}}\\frac{1}{n^{2}}=frac{1}{2^{2}}-frac{1}{lambda R}\\frac{1}{n^{2}} = frac{lambda R-2^{2}}{2^{2}lamda R}\\n^{2} = frac{2^{2}lambda R}{lambda R-2^{2}}\\n = sqrt{frac{2^{2}lambda R}{lambda R-2^{2}}}[/latex]
