First slide

The optical instruments

Question

A simple telescope, consisting of an objective of focal length 60 cm and a single eye lens of focal length 5 cm is focussed on a distant object is such a way that parallel rays comes out from the eye lens. If the object subtends an angle 2° at the objective, the angular width of the image

Moderate

A

10°
B

24°
C

50°
D

$\frac{1}{6} °$

Solution

$Since m = \frac{f_{o}}{f_{e}} Also m = \frac{Angle subtended}{Angle subtended} by the image ∴ \frac{f_{o}}{f_{e}} = \frac{α}{β} \Rightarrow α = \frac{f_{0} \times β}{f_{e}} = \frac{60 \times 2}{5} = 24^{\circ}$

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