Question

An object is placed perpendicular to the principal axis of a lens of focal length f at a distance of 20 cm from the lens which produces an image at a distance 12 cm on the same side of the object in front of the lens. Find the focal length of the lens and state the type of lens.
[4 MARKS]

Answer 1

Lens formula: 1 Mark
focal length: 2 Marks
Type of lens: 1 Mark

Here, Focal length of a concave lens, f =?
Distance of the object, u = - 20 cm
Distance of the image, v = - 12 cm
Using lens formula,$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{1}{-12}-\frac{1}{-20}$
$\therefore \frac{1}{f}=\frac{-1}{30}$
$\Rightarrow f=-30cm$
Hence the focal length of the lens is 30 cm. Negative sign indicates that it is a concave lens.