In displacement method, there are two position of a lens for whic… - Vidyadip

MCQ

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 ......... $cm$

A
$40$
B
$\frac{40}{3}$
C
$80$
✓
$\frac{80}{3}$

✓

Answer

Correct option: D.

$\frac{80}{3}$

d
(d)

Displacement method is based on principle of reversibility. The image distance for 1st position of lens will be same as object position with IInd position of lens.

$u=-40 \,cm$

$V=+80 \,cm$

$\frac{1}{f}=\frac{1}{80}-\frac{1}{-40}$

$f=\frac{80}{3} \,cm$

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