Question

If an object is kept 20 cm away from lens and image is formed 40 cm away from the lens, then what
is the focal length of the lens?

Answer 1

$\huge\underline{\underline{\bf \orange{Question-}}}$

If an object is kept 20 cm away from lens and image is formed 40 cm away from the lens, then what is the focal length of the lens?

$\huge\underline{\underline{\bf \orange{Solution-}}}$

Let's say lens is Convex

$\large\underline{\underline{\sf Given:}}$

• Object distance from lens (u) = -20cm
• Image distance from lens (v) =+40cm

$\large\underline{\underline{\sf To\:Find:}}$

• Focal Lenght (f)

❏ Lens Formula

❏$\large{\boxed{\bf \blue{\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}}}}$

$\implies{\sf \dfrac{1}{f}=\dfrac{1}{40}-\left(-\dfrac{1}{20}\right) }$

$\implies{\sf \dfrac{1}{f}=\dfrac{1}{40}+\dfrac{1}{20} }$

$\implies{\sf \dfrac{1}{f}=\dfrac{20+40}{40×20} }$

$\implies{\sf \dfrac{1}{f}=\dfrac{60}{800}}$

$\implies{\sf f =\dfrac{800}{60}}$

$\implies{\bf \red{f=13.3cm} }$

$\huge\underline{\underline{\bf \orange{Answer-}}}$

Focal lenght of lens is ${\bf \red{13.3cm}}$.