CBSE Class 12 Physics 2015 Delhi Set 1 Paper

Question : 12 of 26

Marks: +1, -0

(i) A giant refracting telescope has an objective lens of focal length

15 {\m}

. If an eye piece of focal length

1.0 {\cm}

is used, what is the angular magnification of the telescope?
(ii) If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens ? The diameter of the moon is

3.48 × 10^6 {\m}

and the radius of lunar orbit is

3.8 ×

10^8 {\m}

Solution:

(i) Calculation of angular magnification
(ii) Calculation of image of diameter of Moon
Angular Magnification

m\;=\;{f_o}/{f_e}

\;=\;{15}/{10^{-2}}=1500

\;\text " Angular size of the moon "\;\;= (\;{3.48 × 10^6}/{3.8 × 10^8})

\;=\;{3.48}/{3.8} × 10^{-2} \;\text " radian "\;

Angular size of the image

= (\;{3.48}/{3.8} × 10^{-2} × 1500) \;\text " radian "\;

Diameter of the image

=\;{3.48}/{3.8} × 15 × \;\text " focal length of eye peice "\;

\;=\;{3.48}/{3.8} × 15 × 1 {\cm}

\;=13.7 {\cm}

(Also accept alternative correct method.)

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