Question

Example 3. An object is placed at a distance of 20 cm from a concave mirror of focal
length 10 cm. Find the position.
1
Tini
+​

Answer 1

Given:

$\sf{u=-20cm}$

$\sf{f=-10cm}$

$\sf{mirror=concave \:mirror}$

Solution__

We know that:

$\sf{{\dfrac{1}{f}}={\dfrac{1}{v}}+{\dfrac{1}{u}}}$

$\sf{{\dfrac{1}{v}}={\dfrac{1}{20}}-{\dfrac{1}{10}}}$

$\sf{{\dfrac{1}{v}}=-{\dfrac{1}{20}}}$

$\sf{v=-20cm}$

Also,

$\sf{m=-{\dfrac{v}{u}}}$

$\sf{m=-{\dfrac{-20}{-20}}}$

$\sf{m=-1}$

$\sf{\therefore{hi=ho=1}}$

So the image formed will be of 20cm , real and inverted and of the same size as the object.