Question

A a concave lens of focal length 15cm forms an image of 10 cm from the lens how far is the object placed from the lens .....???​

Answer 1

$\large{\underline{\underline{\mathfrak{\green{\bf{Question:-}}}}}}$.

• A concave lens of focal length 15cm from of image of 10cm from the lens how far is the object placed from the lens .

$\large{\underline{\underline{\mathfrak{\bf{Given\:Here:-}}}}}$.

• Focal length of the concave lens f = -15cm

• Image distance from the concave lens v = -10cm

$\large{\underline{\underline{\mathfrak{\bf{Find\:Here:-}}}}}}$.

• Distance of object from the concave lens .

Lens Formula

$\large\boxed{\frac{1}{f}\:=\frac{1}{v}\:-\frac{1}{u}}$

Therefore ,

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

Keep Value of f and v ,

$\implies\frac{1}{u}\:=\frac{1}{-10}\:-\frac{1}{-15}$

$\implies\frac{1}{u}\:=\frac{-1}{10}\:+\frac{1}{15}$

$\implies\frac{1}{u}\:=\frac{-15+10}{15×10}$

$\implies\frac{1}{u}\:=\frac{-5}{150}$

$\implies\frac{1}{u}\:=\frac{-1}{30}$

$\large\boxed{\:u\:=\:-30}$

__________________________

★The Object distance from the concave lens = -30cm

★Therefore the object is placed at 30cm from the concave lens

______________________

Answer 2

Answer:

$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \\ f = \frac{uv}{u - v} \\ 15 = \frac{10u}{u - 10} \\ 15u - 150 = 10u \\ 5u = 150 \\ u = 30cm$