calculate the focal length of the lens

Question

During the experiment student recorded that distance between object and the screen, on which image is observed, is 674 mm. Using the thin lens equation , calculate the focal length of the lens. The value for magnification is −2.15.

Answer 1

The thin lens equation is that 1/d_o + 1/d_i = 1/f (o stands for object and i stands for image: the image value is + if the rays go to the opposite side of the lens from the object and form a real image) and the magnification is -d_i/d_o - -2.15 which indicates that the image is magnified and real (d_i>0). This happens for a converging lens when d_o is outside the focal length and inside 2*f:

1/f = 1/(67.4 cm/2.15) + 1/67.4cm = 1/f f = 21.4 cm

Hope that helps.