The image of a candle flame placed at a distance of 36 cm from a spherical lens, is formed on a screen placed at a distance of 72 cm from the lens. Calculate…

Given,

Object distance (u) = - 36 cm

As, image is formed on a screen then,

Image distance (v) = + 72 cm

Let, focal length be f.

From lens formula,

$$\begin{aligned}\frac{1}{\text f} = \frac{1}{\text v} - \frac{1}{\text u} \\[1 em] \frac{1}{\text f} = \frac{1}{72}-\frac{1}{(-36)}=\frac{1}{72}+\frac{1}{36}= \frac{1+2}{72} \\[1em] \frac{1}{\text f} = \frac{3}{72} \\[1 em] \text f = \frac{72}{3} = + 24 \text { cm}\end{aligned}$$

Now,

$\text {Power} = \frac{1}{\text {f (in metre)}} = \frac{1}{24\times 10^{-2}}=\frac{100}{24} ≈ 4.17\ \text D$