A source of light is located at a distance 1 = 90 cm from
a screen. A thin converging lens provides the sharp image of the
source when placed between the source of light and the screen at two
positions. Determine the focal length of the lens if
(a) the distance between the two positions of the lens is Al =
= 30 cm;
(b) the transverse dimensions of the image at one position of the
lens are ri = 4.0 greater than those at the other position.