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CBSE Class 12 Physics 2020 Outside Delhi Set 1 Paper

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Question : 34
Total: 37
Two objects P and Q when placed at different positions in front of a concave mirror of focal length 20‌cm, form real images of equal size. Size of object P is three times the size of object Q. If the distance of P is 50‌cm from the mirror, find the distance of Q from the mirror.
Solution:  
Mirror formula : ‌
1
v
+‌
1
u
=‌
1
f

‌ Magnification ‌=m=‌
h′
h
=‌
v
u

For object P :
up‌=−50‌cm
f‌=−20‌cm
‌
1
vp
+‌
1
up
‌=‌
1
f

‌ Or, ‌‌‌‌
1
vp
+‌
1
−50
‌=‌
1
−20

‌∴vP=−‌
100
3
‌cm
‌mP=‌
hP′
hP
=‌
vP
uP
=‌
−
100
3
−50
=‌
2
3

For object Q :
mQ‌=‌
hQ′
hQ
=‌
vQ
uQ

Or, ‌
hp′
1
3
hP
‌=‌
vQ
uQ

∴vQ=uQ
‌‌‌
1
vQ
+‌
1
uQ
‌=‌
1
f

‌
1
2uQ
+‌
1
uQ
‌=‌
1
−20

∴ uQ‌=−30‌cm
So, the distance of Q from mirror is 30‌cm .
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