પ્રતિબિંબ વચ્ચેનું અંતર $15 \mathrm{~cm}$ હોય તો અરીસાની કેન્દ્ર લંબાઇ_________થશે.
\( 2=\frac{-(15-\mathrm{u})}{-\mathrm{u}} \)
\( 2 \mathrm{u}=15-\mathrm{u} \)
\( 3 \mathrm{u}=15 \Rightarrow \mathrm{u}=5 \mathrm{~cm} \)
\( \mathrm{v}=15-\mathrm{u}=15-5=10 \mathrm{~cm} \)
\( \frac{1}{\mathrm{f}}=\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}} \)
\( =\frac{1}{10}+\frac{1}{(-5)}=\frac{1-2}{10}=\frac{-1}{10} \)
\( \mathrm{f}=-10 \mathrm{~cm}\)