2 Marks Questions

Question 12 Marks

Differentiate between primary rainbow and secondary rainbow.

Answer

Primary rainbow	Secondary rainbow
1. It forms between about 40° and 42° from the antisolar point.	It is about 10° further out from the antisolar point than the primary rainbow.
2. Width of primary rainbow is 1/2 as secondary rainbow.	It is turice as wide than primary rainbow.

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Question 22 Marks

Draw the ray diagram of an astronomical telescope showing image formation in the normal adjustment position. Write the expression for its magnifying power.

Answer

$m=\frac{\beta}{\alpha}=\frac{- AB / F _e}{ AB / f_0}=\frac{f_0}{f_e}$

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Question 32 Marks

Draw a labelled ray diagram to show image formation by a compound microscope and write the expression for its resolving power.

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Question 42 Marks

Two thin lenses of powers 5D and 2D are placed coaxially at a distance 20 cm apart. Find the focal length and power of the combination of lenses.

Answer

Given : $P_{1}=5 D, P_{2}=2 D$ and $d=20$ cm
$\therefore$ $f_{1}=\frac{1}{P_{1}}=(\frac{1}{5})m=20 cm$
and $f_{2}=\frac{1}{P_{2}}=(\frac{1}{2})m=50 cm$
If the focal length of the combination is F then :
$\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1 f_2}$
$=\frac{1}{20 cm}+\frac{1}{50 cm}-\frac{20 cm}{20 cm \times 50 cm}$
$=\frac{1}{20 cm}$
$\Rightarrow \ F =20 cm$

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