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NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions

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Question : 31
Total: 38
What should be the distance between the object in previous question and the magnifying glass if the virtual image of each square in the figure is to have an area of 6.25 mm2. Would you be able to see the squares distinctly with your eyes very close to the magnifier?
Solution:  
Now we want the area of square shaped virtual image as 6.25 mm2.
So, each side of image is I = √6.25 = 2.5 mm (Linear magnification)
For the given magnifying lens of focal length 10 cm we can calculate the required position of object.
Magnification m =
I
O
 =
v
u
I
O
 =
v
u
 ⇒
2.5mm
1mm
 =
v
u
 , So, v = 2.5 u
Lens formula
1
v
 =
1
u
 =
1
f
 or
1
2.5u
 -
1
u
 =
1
+10
u = – 6 cm and v = – 2.5 × u = – 15 cm.
Thus the required virtual image is closer than normal near point.
Thus the eye cannot observe the image distinctly.
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