A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm The focal length of the lens is

Question

A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm The focal length of the lens is (A) (770/36) cm (B) (77/36) cm (C) (36/770) cm (D) (360/77) cm

Tardigrade Admin · Accepted Answer

Here, distance between object and screen D=90 cm Distance between two locations of convex lens d=20 cm , f= ? As, f=(D2-d2/4 D)=(902-202/4 × 90) =(110 × 70/360)=(770/36) cm

Q. A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm The focal length of the lens is

$\frac{770}{36} c m$

$\frac{77}{36} c m$

$\frac{36}{770} c m$

$\frac{360}{77} c m$

Here, distance between object and screen $D = 90 c m$
Distance between two locations of convex lens
$d = 20 c m, f =$ ?
As, $f = \frac{D ^{2} - d ^{2}}{4 D} = \frac{9 0 ^{2} - 2 0 ^{2}}{4 \times 90}$
$= \frac{110 \times 70}{360} = \frac{770}{36} c m$

Q. A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm The focal length of the lens is

36770​cm

3677​cm

77036​cm

77360​cm

Here, distance between object and screen D=90cm Distance between two locations of convex lens d=20cm,f= ? As, f=4DD2−d2​=4×90902−202​ =360110×70​=36770​cm

$\frac{770}{36} c m$

$\frac{77}{36} c m$

$\frac{36}{770} c m$

$\frac{360}{77} c m$

Here, distance between object and screen $D = 90 c m$
Distance between two locations of convex lens
$d = 20 c m, f =$ ?
As, $f = \frac{D ^{2} - d ^{2}}{4 D} = \frac{9 0 ^{2} - 2 0 ^{2}}{4 \times 90}$
$= \frac{110 \times 70}{360} = \frac{770}{36} c m$