Question

If the object is placed at a distance of 8 cm from a converging lens, a real image is produced at 12cm. What is the focal length of the lens?

Answer 1

$$\dfrac{1}{d_o}+\dfrac{1}{d_i}=\dfrac{1}{f},$$ $$f=\dfrac{1}{\dfrac{1}{d_o}+\dfrac{1}{d_i}}=\dfrac{1}{\dfrac{1}{8\ cm}+\dfrac{1}{12\ cm}}=4.8\ cm.$$