CBSE Class 10 Science 2015 Term II Delhi Set 2

Question : 11 of 11

Marks: +1, -0

The image of a candle flame placed at a distance of

30 {\cm}

from a spherical lens is formed on a screen placed on the other side of the lens at a distance of

60 {\cm}

from the optical centre of the lens. Identify the type of lens and calculate its focal length. If the height of the flame is

3 {\cm}

, find the height of its image.

Solution:

Given:

h_1=+3 {\cm}, u=-30 {\cm}

v=+60 {\cm}, f=\;\text " ?, "\; h^{'}=\;\text " ? "\;

\;{1}/{f}=\;{1}/{v}-\;{1}/{u}=\;{1}/{60}-\;{1}/{-30}

\;{1}/{f}=\;{+3}/{60}

∴ \;\; f=+20 {\cm}

Therefore, the lens is convex.

h^{'}\;=\;{v}/{u} × h=\;{+60 {\cm}}/{-30 {\cm}} × 3 {\cm}

h^{'}\;=-6 {\cm}

Here, negative sign indicates that the image formed is inverted. Therefore, height of image of candle flame is

6 {\cm}

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