Question

An object placed $45cm$ from a lens forms an image on a screen placed $90cm$ on other side of the lens Identify the type of the lens and find its focal length

Answer 1

Here image formed on screen do it is a Real image so,lens is then

$u=-45cm$

$V=90cm$

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{f}=\frac{1}{90}-\frac{1}{-45}=\frac{1}{90}+\frac{1}{45}=\frac{1+2}{90}$

$\frac{1}{f}=\frac{3}{90}$

$f=\frac{90}{3}=30cm$