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In a convex lens a real image of magnification 2 becomes virtual image of magnification 4. when the object is shifted by 1 m .calculate tha power of the lens?
magnification = -v/u where v=image distance,u=object distance.
1/v - 1/u =1/f where f=focal length.If focal length is known,power=1/focal length.
1st case
magnification =2.
1/v - 1/u = 1/f
1-v/u = v/f
1+2=v*power
power=3/v
2nd case
Given power is shifted by 1m,magnification=4.
power=3/v + 1.
1+4=v*power
power=5/v
3/v + 1=5/v
3+v=5
v=2m.
power of the lens=3/2 = 1.5 dioptre.


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