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  4. In displacement method, there are two position of a lens for which we get clear image. First position of the lens is at 40 cm from object and second is at 80 cm, the focal length of lens is

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Q. In displacement method, there are two position of a lens for which we get clear image. First position of the lens is at $40\, cm$ from object and second is at $80\, cm$, the focal length of lens isPhysics Question Image

Ray Optics and Optical Instruments

Solution:

Displacement method is based on principle of reversibility. The image distance for 1 st position of lens will be same as object position with Ilnd position of lens.
image
$u=-40\, cm$
$V=+80 \,cm$
$\frac{1}{f}=\frac{1}{80}-\frac{1}{-40}$
$f=\frac{80}{3} \,cm$