CBSE Class 12 · 2009 · Geometrical OpticsVery Short Answer

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You are given the following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope? (Lens

L_{1}

: power

3 D

, aperture

8 cm

;

L_{2}

6 D

1 cm

;

L_{3}

10 D

1 cm

Solution

The objective must have a \emph{large aperture} (more light, higher resolving power) and a \emph{large focal length};

L_{1}

has the biggest aperture (

8

cm) and smallest power (

3

\Rightarrow f = 33.3

cm). The eyepiece must have a \emph{short focal length};

L_{3}

has the highest power (

10

\Rightarrow f = 10

cm).

Objective = L_{1}, Eyepiece = L_{3}

CBSE Class 12 Board · 2009 · Subjective

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