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CBSE Class 10 Science 2016 Term II Outside Delhi Set 3

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Question : 11
Total: 11
(a) Define focal length of a divergent lens.
(b) A divergent lens of focal length 30‌cm forms the image of an object of size 6‌cm on the same side as the object at a distance of 15‌cm 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=−30‌cm,u= ?, v=−15‌cm, ho=6‌cm,hi= ?We know that,‌
1
f
=‌
1
v
−‌
1
u
or‌
1
u
=‌
1
v
−‌
1
f
⇒‌‌u‌=‌
vf
f−v
=‌
−15×−30
−30+15

=‌
450
−15
=−30‌cm.
Now,m=‌
hi
ho
=‌
v
u
⇒‌‌hi=‌
v
u
×ho=‌
−15
−30
×6=3‌cm.
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