a
અસરકારક કેન્દ્રલંબાઇ ને \(\frac{1}{F}\,\, = \,\,\frac{2}{{{{f}_{l}}}}\, + \,\,\frac{1}{{{{f}_m}}}\)  વડે અપાય છે 

પરંતુ \(\frac{1}{{{{f}_{l}}}}\,\, = \,\,{\text{(1}}{\text{.5 - 1)}}\,\,\left( {\frac{{\text{1}}}{\infty }\,\, + \,\,\frac{1}{{30}}} \right){\text{ }}\, = \,\,{\text{ }}\frac{{\text{1}}}{{{\text{60}}}}\) અથવા \(\,\frac{2}{{{{f}_{l}}}}\,\, = \,\,\frac{1}{{30}}\) ફરીથી \(R\,\, = \,\,30\,\,cm\,\,\,\,\) 

\({{f}_m}\, = \,\,\frac{R}{2}\,\, = \,\,15\,\,\,cm\)

હવે,\(\frac{1}{F}\,\, = \,\,\frac{1}{{30}}\,\, + \,\,\frac{1}{{15}}\) અથવા \(\frac{1}{F}\,\, = \,\,\frac{{1 + 2}}{{30}}\,\, = \,\,\frac{3}{{30}}\,\, = \,\,\,\frac{1}{{10}}\)  અથવા  \(F\,\, = \,\,10\,\,\,cm\)

માંગેલું અંતર  \(2\,\, \times \,\,10\,\,\, = \,\,20\,\,\,cm\)