Question

11. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far
is the object placed from the lens? Draw the ray diagram​

Answer 1

Given :-

• Image distance (v) = - 10 cm

• Focal length (f) = - 15 cm

To find :-

• Object distance (u) = ?

Solution :-

By using lens formula :-

$\rm\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$

$\rm\longrightarrow\dfrac{1}{-10}-\dfrac{1}{u}=\dfrac{1}{-15}$

$\rm\longrightarrow-\dfrac{1}{u}=-\dfrac{1}{15}+\dfrac{1}{10}$

$\rm\longrightarrow-\dfrac{1}{u}=\dfrac{-2+3}{30}$

$\rm\longrightarrow-\dfrac{1}{u}=\dfrac{1}{30}$

$\rm\longrightarrow\:u=-30\:cm$

Hence,the object distance will be 30 cm from the concave mirror.The minus sign indicate that the object is on its left side.

More Information :-

concave lens :- A concave lens is thin in the middle but thicker at the edge.

• A concave lens diverges a parallel beam of light rays.

• The principal focus of a concave lens is a point on its principal axis from which light rays , originally parallel to the axis appear to diverge through the concave lens.