An optical instrument uses an eye-lens of power $16\ \mathrm{D}$ and an objective lens of power $50\ \mathrm{D}$, and has a tube length of $16.25\ \mathrm{cm}$. Name the optical instrument and calculate its magnifying power if it forms the final image at infinity.

CBSE Class 12 · 2017 · Geometrical OpticsShort Answer

An optical instrument uses an eye-lens of power

16 D

and an objective lens of power

50 D

, and has a tube length of

16.25 cm

. Name the optical instrument and calculate its magnifying power if it forms the final image at infinity.

Answer: The instrument is a compound microscope; its magnifying power $\approx 32.5$ .

Focal lengths:

f_{o} = \frac{100}{50} = 2 cm

f_{e} = \frac{100}{16} = 6.25 cm

. The short focal lengths identify a \emph{compound microscope}.
For the final image at infinity:

m = \frac{L}{f _{o}} \cdot \frac{D}{f _{e}} = \frac{16.25}{2} \times \frac{25}{6.25} = 8.125 \times 4 = 32.5

CBSE Class 12 Board · 2017 · Subjective