Two thin lenses may be combined so as to

What condition must be satisfied by two thin lenses kept in contact so that they form an achromatic doublet, i.e., a combination free from chromatic aberration ?

When two thin lenses are kept in contanct, the focal length of the combination is

When two thin lenses are kept in contanct, the focal length of the combination 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 having optical powers of -10 D and + 6 D are placed in contact with each other. The focal length of the combination is :