Question

Magnjfying power of a simple microscope A is 1.25 less than that of a simple microscope B.If the power of tehe lens used in B is 25D. find the power of the lens used in A.Dist. of distint vision is 25cm.

Answer 1

$$\begin{gathered} \mathrm{For}\mathrm{B}, \\ \mathrm{Magnifying}\mathrm{power},\mathrm{M}=1+\frac{\mathrm{D}}{\mathrm{f}} \\ \mathrm{M}=1+0.25\times 25 \\ \mathrm{Since}\mathrm{D}=25\mathrm{cm}=0.25\mathrm{m}(\mathrm{given}\mathrm{in}\mathrm{question}) \\ \mathrm{and}\mathrm{power}=25\mathrm{D}\mathrm{i}.\mathrm{e}\frac{1}{\mathrm{f}(\mathrm{in}\mathrm{metres})}=\mathrm{P}=25 \\ \mathrm{So},\mathrm{M}=1+6.25=7.25 \\ \mathrm{For}\mathrm{A}, \\ \mathrm{M}\text{'}=\mathrm{M}-1.25 \\ =7.25-1.25=6 \\ \mathrm{So},\mathrm{M}\text{'}=1+0.25\times \mathrm{P}\text{'} \\ \mathrm{or}6-1=1+0.25\times \mathrm{P}\text{'} \\ \mathrm{or}\mathbf{}\mathbf{P}\mathbf{\text{'}}\mathbf{=}\mathbf{20}\mathbf{}\mathbf{D} \end{gathered}$$