Question

Calculate the position of image formed when an object is place 20cm and 28cm respectively in front of a convex lens. (Focal length of the convex lens is 15cm)

Answer 1

a) We can find the position of image from the lens equation:

$$\dfrac{1}{d_o} + \dfrac{1}{d_i} = \dfrac{1}{f},$$ $$\dfrac{1}{d_i} = \dfrac{1}{f} - \dfrac{1}{d_o},$$ $$d_i = \dfrac{1}{\dfrac{1}{f} - \dfrac{1}{d_o}} = \dfrac{1}{\dfrac{1}{15 \ cm} - \dfrac{1}{20 \ cm}} = 60 \ cm.$$

b) We can find the position of image (when the object distance from the lense is 28 cm) from the same equation:

$$d_i = \dfrac{1}{\dfrac{1}{f} - \dfrac{1}{d_o}} = \dfrac{1}{\dfrac{1}{15 \ cm} - \dfrac{1}{28 \ cm}} = 32 \ cm.$$

Answer:

a) $d_i = 60 \ cm.$

b) $d_i = 32 \ cm.$