The magnifying power of a microscope with an objective of 5 mm focal length is 400 . The length of its tube is 20 cm. Then the focal length of the eye-piece is

Question

The magnifying power of a microscope with an objective of 5 mm focal length is 400 . The length of its tube is 20 cm. Then the focal length of the eye-piece is (A) 200 cm (B) 160 cm (C) 2.5 cm (D) 0.1 cm

Tardigrade Admin · Accepted Answer

If nothing is said then it is considered that final image is formed at infinite and m ∞ ((L∞-fo-fe) ⋅ D/fo fe) ≃ (L D/f0 fe) ⇒ 400=(20 × 25/0.5 × fe) ⇒ fe=2.5 cm

Q. The magnifying power of a microscope with an objective of $5 mm$ focal length is $400$ . The length of its tube is $20 c m$ . Then the focal length of the eye-piece is

200 cm

160 cm

2.5 cm

0.1 cm

If nothing is said then it is considered that final image
is formed at infinite and $m_{\infty} \frac{( L _{\infty} - f _{o} - f _{e} ) \cdot D}{f _{o} f _{e}} ≃ \frac{L D}{f _{0} f _{e}}$
$\Rightarrow 400 = \frac{20 \times 25}{0.5 \times f _{e}}$
$\Rightarrow f_{e} = 2.5 c m$

Q. The magnifying power of a microscope with an objective of 5mm focal length is 400 . The length of its tube is 20cm. Then the focal length of the eye-piece is