Two thin lenses of focal lengths 20cm and 25cm are placed in contact. The effective power of the combination is

Two thin lenses of focal lengths 20 cm and 25 cm are placed in contact. The effective power of the combinations is

Two thin lenses of focal lengths 20 cm and 25 cm are placed in a contact. The effective power of the combination is

Two thin lenses of focal lengths 20 cm and 25 cm are placed in a contact. The effective power of the combination is

Two thin lenses of focal lengths 20 cm and 25 cm are placed in a contact. The effective power of the combination is

Two lenses of focal length 10 cm and -20 cm are placed in contact .What is the total power of the combination?

Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with other. The combination has a focal length equal to

Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with other. The combination has a focal length equal to