Two thin lenses, one of focal length 60 cm and the other of focal length -20 cm are put in contact,

Question

Two thin lenses, one of focal length 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is

Professor PC Thomas Classes · Answer

$1/F = 1/f_1 + 1/f_2 = 1/60 - 1/20 = -2/60$. $F = -30 	ext{ cm}$.