The distance between an object and a screen is 100 cm A lens can produce real image of the object on the screen for two different positions between the screen and the object. The distance between these two positions is 40 cm If the power of the lens is close to (( N /100)) D where N is an integer, the value of N is .

Question

The distance between an object and a screen is 100 cm A lens can produce real image of the object on the screen for two different positions between the screen and the object. The distance between these two positions is 40 cm If the power of the lens is close to (( N /100)) D where N is an integer, the value of N is . (A)  (B)  (C)  (D)

Tardigrade Admin · Accepted Answer

Using displacement method f =( D 2- d 2/4 D ) Here, D=100 cm d =40 cm f =(1002-402/4(100))=21 cm P =(1/ f )=(100/21) D ( N /100)=(100/21) N =476

Using displacement method f=4DD2−d2​ Here, D=100cm d=40cm f=4(100)1002−402​=21cm P=f1​=21100​D 100N​=21100​ N=476

Using displacement method
$f = \frac{D ^{2} - d ^{2}}{4 D}$
Here, $D = 100 c m$
$d = 40 c m$
$f = \frac{10 0 ^{2} - 4 0 ^{2}}{4 ( 100 )} = 21 c m$
$P = \frac{1}{f} = \frac{100}{21} D$
$\frac{N}{100} = \frac{100}{21}$
$N = 476$