CBSE Class 10 Science 2016 Term II Outside Delhi Set 3

Question : 11 of 11

Marks: +1, -0

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

30 {\cm}

forms the image of an object of size

6 {\cm}

on the same side as the object at a distance of

15 {\cm}

from its optical centre. Use lens formula to determine the distance of the object from the lens and the size of the image formed.
(c) Draw a ray diagram to show the formation of image in the above situation.

Solution:

(a) The point from which ray of light parallel to principal axis after refraction, appear to diverge is called principal focus of a divergent lens and the distance between optical centre and this focus is called focal length of a divergent lens.
(b) Given,

f=-30 {\cm}, u=

v=-15 {\cm}

h_o=6 {\cm}, h_i=

?We know that,

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

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

⇒ \;\; u\;=\;{v f}/{f-v}=\;{-15 ×-30}/{-30+15}

=\;{450}/{-15}=-30 {\cm} .

Now,

m=\;{h_i}/{h_o}=\;{v}/{u}

⇒ \;\; h_i=\;{v}/{u} × h_o=\;{-15}/{-30} × 6=3 {\cm} .

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