Question

An object is placed at a distance of $40cm$ from a concave mirror of focal length $15cm$. If the object is displaced through a distance of $20cm$ towards the mirror, the displacement of the image will be

• A. $30cm$ away from the mirror
• B. $36cm$ away from the mirror
• C. $30cm$ towards the mirror
• D. $36cm$ towards the mirror

Answer 1

The correct option is B $36cm$ away from the mirror


$v_1=\frac{uf}{u-f}=\frac{\left(-40\right)\left(-15\right)}{-40+15}=\frac{600}{-25}=-24cm$


$v_2=\frac{uf}{u-f}=\frac{\left(-20\right)\left(-15\right)}{-20+15}=\frac{300}{-5}=-60cm$
Displacement of image = $v_2-v_1=-36cm$
i.e $36cm$ away from the mirror