Question

An extended object is placed at a distance of 5.0 cm from a convex lens of focal length 8.0 cm. (a) Draw the ray diagram (to the scale) to locate the image and from this, measure the distance of the image from the lens. (b) Find the position of the image from the lens formula and see how close the drawing is to the correct result.

Answer 1

Given,
Object distance, (u) = 5.0 cm
Focal length (f) of convex lens = 8.0 cm

(a)


(b)
Using lens formula:

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
Where v is the image distance,
on putting the given values we get:

$$\begin{gathered} \frac{1}{v}-\frac{1}{\left(-5\right)}=\frac{1}{8} \\ \Rightarrow \frac{1}{v}=\frac{1}{8}-\frac{1}{5} \\ \Rightarrow \frac{1}{v}=\frac{-3}{40} \\ \therefore v=-13.3\mathrm{cm} \end{gathered}$$

Hence, the position of the image from the lens is 13.3 cm.