• # question_answer Using the data given below, state which two of the given lenses will you prefer to construct a best possible (i) telescope        (ii) compound microscope. Also, indicate which of the selected lenses is to be used as an objective and as an eye lens in each case. Lens Power (P) Aperture (A) ${{L}_{1}}$ 6 D 1 cm ${{L}_{2}}$ 3 D 8 cm ${{L}_{3}}$ 10 D 1 cm

Power of lens, $P=\frac{1}{f(in\,metre)}$             $\Rightarrow$   $f=1/P$ or $f(in\,cm)=100/P$ Focal length of three lenses, ${{f}_{1}}=16.67\,cm,$ ${{f}_{2}}=33.33\,cm$ and ${{f}_{3}}=10\,cm$  (i) For telescope Objective should be of large aperture and large focal length, so lens ${{L}_{2}}$ is fit for objective. Eye lens should be of small aperture and small focal length. So, lens ${{L}_{3}}$ is fit for making eye lens. (ii) For compound microscope Objective must be of small aperture and small focal length and therefore, lens ${{L}_{3}}$ can be used for objective. The lens ${{L}_{1}}$ of small aperture and larger focal length is fit for making eye lens.