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Q1. A concave lens has focal length 20cm.
At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens?
Also calculate the size of the
image formed.

Q2. A convex lens has a focal length of 30 cm. Calculate at what distance should the object be placed from the lens so that it forms an image at 60 cm on the other side of the lens?. Find the magnification produced by the lens in this case.

 

1. 1/u=1/v -1/f

          = -1/15 +1/20

          =-4+3/60

           =-1/60

u=-60cm

H=v/u *h

 =-15/-60 * 5

=+1.25 cm

2. 1/u =1/v-1/f

          =1/60 -1/30

          = 1-2/60

           =-1/60

u= -60cm

m=v/u

m=60/-60

m=-1

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why is v negative can u plz explan
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How can a concave lens produce a magnified image?
 
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why is v -ve?
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every thing is correct except for the magnification step
m=v/u=-15/60=1/-4=-1.25
THUS, h'=hX-0.25=-1.25
 
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f = – 20 cm.
h = 5 cm.
v = –15 cm.
1v−1u=1

1u=1v−1f=−115−(−120)
=−160
Therefore,  u = – 60 cm.
h′h=vu
h′=5×−15−60=54cm
Object must be placed 60 cm. Size of image is 1.25 cm.

   
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Why v is negative.i think it's wrong 
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u=60 m=5/4
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What is the answer of 2 part
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1 part we all know
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f=-20cm
v=-15cm
h=5cm

By lens formula,
1/u=1/v-1/f
     =-1/15+1/20
     =-1/60

So u=-60 cm
Magnification=v/u
                       =-15/-60=0.25
Since
Hi/Ho=v/u=m,
Hi=m*Ho
    =0.25*5cm=1.25cm
So Height of image or size of image=1.25cm
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Hope answer is clear

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Thanks
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