CBSE Class 10 Science 2016 Term II Outside Delhi Set 2

Question : 12 of 12

Marks: +1, -0

(a) Define focal length of a spherical lens.
(b) A divergent lens has a focal length of

30 {\cm}

. At what distance should an object of height

5 {\cm}

from the optical centre of the lens be placed so that its image is formed

15 {\cm}

away from the lens? Find the size of the image also.
(c) Draw a ray diagram to show the formation of image in the above situation.

Solution:

(a) The distance between the optical centre and focus of a spherical lens is called focal length.
(b) Given :

f=-30 {\cm}, h_i=5 {\cm}, h_i=

v=-15 {\cm}, u=\;\text " ? "\;

We know that,
or

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

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

u\;=\;{v f}/{f-v}

\;=\;{-15 ×-30}/{-30+15}=-30 {\cm} .

⇒ \;\; u=\;{v f}/{f-v}

Now,

\;\text " Now, "\;\; \;\; m=\;{v}/{u}=\;{h_i}/{h_0}

⇒ h_i=\;{v}/{u} × h_0=\;{-15}/{-30} × 5=2.5 {\cm} .

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