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 |
Answer:
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.
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