A needle placed 45 cm from a lens forms an image on the screen placed 90 cm on the other side of the lens. What is the size of image if the size of needle is 5 cm ? And what is the focal length of lens?

Question

A needle placed 45 cm from a lens forms an image on the screen placed 90 cm on the other side of the lens. What is the size of image if the size of needle is 5 cm ? And what is the focal length of lens? (A) 10 cm , 30 cm (B) -10 cm , 30 cm (C) 100 cm , 10 cm (D) 1 m , 100 cm

Tardigrade Admin · Accepted Answer

(1/f)=(1/v)-(1/u) ⇒ (1/90)+(1/45) (1/f)=(3/90) f=30 cm (I/O)=(-v/u) (I/5)=(-90/-45) ⇒ I=10 cm

Q. A needle placed $45 c m$ from a lens forms an image on the screen placed $90 c m$ on the other side of the lens. What is the size of image if the size of needle is $5 c m ?$ And what is the focal length of lens?

10 cm , 30 cm

-10 cm , 30 cm

100 cm , 10 cm

1 m , 100 cm

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\Rightarrow \frac{1}{90} + \frac{1}{45}$
$\frac{1}{f} = \frac{3}{90}$
$f = 30 c m$
$\frac{I}{O} = \frac{- v}{u}$
$\frac{I}{5} = \frac{- 90}{- 45}$
$\Rightarrow I = 10$ cm

Q. A needle placed 45cm from a lens forms an image on the screen placed 90cm on the other side of the lens. What is the size of image if the size of needle is 5cm? And what is the focal length of lens?