[" 67.Three lenses in contact have a combi- "],[" ned focal length of "12cm" .When the third "],[" lens is removed,the combined focal length "],[" is "60/7cm" .The third lens is "]

Three lenses in contact have a combined focal length of 12 cm. When the third lens is removed, the combined focal length is (60)/(7) cm. The third lens is

Three lenses in contact have a combined focal length of 12 cm. When the third lens is removed, the combined focal length is (60)/(7) cm. The third lens is

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,

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

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

Two thin lenses, aone convex of focal length 10 cm and other concave lens are kept in contact to form a composite lens of focal length 13 cm . If the combination acts as a converging lense the focal length of second lens is

Two thin lenses, aone convex of focal length 10 cm and other concave lens are kept in contact to form a composite lens of focal length 13 cm . If the combination acts as a converging lense the focal length of second lens is