First slide

The optical instruments

Question

An astronomical telescope having focal length of objective 300 cm and of eyepiece 5 cm. It is focused to see an object 20 km from the objective. If final image is formed at infinity, then:

Easy

A

Length of tube is 310 cm
B

Length of tube is 295 cm
C

Angular magnification is 60
D

Angular magnification is 100

Solution

$\begin{array}{l} L = f_{o} + f_{e} = 305 c m \\ M = \frac{- f_{o}}{f_{e}} = - \frac{300}{5} = - 60 \end{array}$
Negative sign indicates inverted image.

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