c
લેન્સ મેક્સના સૂત્ર પ્રમાણે \(\frac{1}{{{f}}}\,\, = \,\,\,(\mu  - 1)\,\,\left[ {\frac{1}{{{R_1}}} - \frac{1}{{{R_2}}}} \right]\)  સાથે  \(\mu \,\, = \,\,\frac{{{\mu _L}}}{{{\mu _M}}}\,\,\,\,\,\therefore \,\,\frac{1}{{{{{f}}_a}}}\,\, = \,\,\left[ {\frac{{(3/2)}}{1} - 1} \right]\,\,K\)

અને \(\frac{1}{{{{{f}}_w}}}\, = \,\,\left[ {\frac{{(3/2)}}{{(4/3)}} - 1} \right]\,\,K\)  સાથે \(K\,\, = \,\,\left[ {\frac{1}{{{R_1}}} - \frac{1}{{{R_2}}}} \right]\,\,\,\)

\( \Rightarrow \,\,\frac{{{{{f}}_w}}}{{{{{f}}_a}}}\,\, = \,\,\left[ {\frac{8}{K}} \right]\,\, \times \,\,\left[ {\frac{K}{2}} \right]\,\,\, = \,\,\,4\,\,\,\,\,\)

\(\therefore \,\,{{{f}}_w} = \,\,4\,\,\, \times \,\,0.3\,\, = \,\,1.2\,\,m\)