Question

An object placed 12 cm to the left of a converging lens forms an image on a screen 36 cm to the right of the lens. Calculate the focal length of the lens.

Answer 1

We can find the focal length of the lens from the lens equation:

$$\dfrac{1}{d_o}+\dfrac{1}{d_i}=\dfrac{1}{f},$$ $$f=\dfrac{1}{\dfrac{1}{d_o}+\dfrac{1}{d_i}}=\dfrac{1}{\dfrac{1}{12\ cm}+\dfrac{1}{36\ cm}}=9\ cm.$$

Answer:

$f=9\ cm.$