Two lenses of focal lengths 24 cm and – 36 cm are held in contact. The image of an object at infinity will be formed by the combination at;

48cm 72cm 36cm 100cm

72cm

1/f = 1/f1 + 1/f2  f1 = 24cm f2 = -36cm 1/f = (1/24)-(1/36) = (3-2)/72 = 1/72   f = 72 (1/v)- (1/∞) Hence v=f=72cm