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

Question

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 (A) 40 cm (B) (40/3) cm (C) 80 cm (D) (80/3) cm

Tardigrade Admin · Accepted Answer

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.

u = - 40 c m

V = + 80 c m

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

f = \frac{80}{3} c m

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

$40 c m$

$\frac{40}{3} c m$

$80 c m$

$\frac{80}{3} c m$

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.

$u = - 40 c m$
$V = + 80 c m$
$\frac{1}{f} = \frac{1}{80} - \frac{1}{- 40}$
$f = \frac{80}{3} c m$

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

40cm

340​cm

80cm

380​cm