First slide

The optical instruments

Question

In Gallilean telescope, if the powers of an objective and eye lens are respectively +1.25 D and – 20 D, then for relaxed vision, the length and magnification will be

Moderate

A

$21 .25 cm and 16$
B

75 cm and 20
C

75 cm and 16
D

8.5 cm and 21.25

Solution

$f_{o} = \frac{1}{1 .25} = 0 .8 m and f_{e} = \frac{1}{- 20} = - 0 .05 m ∴ |L_{co}| = |f_{o}| - |f_{e}| = 0 .8 - 0 .05 = 0 .75 m = 75 cm and |m_{\infty}| = \frac{f_{0}}{f_{e}} = \frac{0 .8}{0 .05} = 16$

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