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CBSE Class 10 Science 2017 Delhi Term II Set 2 Paper

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Question : 9
Total: 9
A 3‌cm tall object is placed 18‌cm in front of a concave mirror of focal length 12‌cm. At what distance from the mirror should a screen be placed to see a sharp image of the object on the screen. Also calculate the height of the image formed.
Solution:  
Using mirror equation :
u=−18‌cm,f=−12‌cm,v=?,h1=+3‌cm,h2=?
‌
1
v
+‌
1
u
=‌
1
f
⇒‌‌‌
1
v
−‌
1
18
‌=−‌
1
12

‌
1
v
‌=−‌
1
12
+‌
1
18

⇒‌‌‌
1
v
‌=‌
−3+2
36
=‌
−1
36

v‌=−36‌cm
Thus, to obtain a sharp image of the object, the screen should be placed at a distance of 36‌cm, in front of the mirror.
Now,
m=‌
−v
u
=‌
h2
h1

m=−(‌
−36
−18
)=−2
or
−2‌=‌
h2
3

h2‌=−6‌cm
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