Lens | Focal length | Aperture |
A | 100 cm | 10 cm |
B | 100 cm | 5 cm |
C | 10 cm | 2 cm |
D | 5 cm | 2 cm |
Answer:
For constructing astronomical telescope, lens A should be used as objective because of its large focal length and large aperture. Lens D should be used as its eyepiece because of its small focal length and small aperture. \[m=\frac{{{f}_{0}}}{{{f}_{e}}}=\frac{100}{5}=20\] Normal length \[=\text{ 1}00\text{ }+\text{ 5 }=\text{ 1}0\text{5 cm}\]Advantages of reflecting telescope are: (i) No chromatic aberration. (ii) Bright image because there is no loss of intensity due to reflection and absorption by objective. Or For constructing compound microscope, the lens D should be used as objective and C as the eyepiece because the lenses should have small focal length and the focal length of the objective should be smaller then that of the eyepiece. Here \[m=-\frac{{{\upsilon }_{0}}}{{{f}_{0}}}\left( 1+\frac{D}{{{f}_{e}}} \right)\] The magnifying power of a compound microscope can be increased by taking both \[{{f}_{0}}\] and \[{{f}_{e}}\] small. For the ray diagram showing the formation of image in a compound microscope, see Fig.
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