question_answer

A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses. The powers diopters of the combination is: [AIPMT (S) 2006]

A) 25

B)                   50                   

C)  infinite    

D)   zero

Correct Answer: D

Solution :

Focal length of combination of lenses placed in contact is

For convex lens,

For concave lens,

Hence,

Hence, power of combination,

Note:    As a convex and a concave lens of same focal lens are placed in contact, so we get achromatism i.e., combination if free from chromatic aberration.