Which one among the following is the correct focal length of a combination of lenses of power 2.5 D and -2.0 D ?
- A.+0.5 m
- B.-0.5 m
- C.+2.0 m
- D.-2.0 m
Show answer
For lenses in contact the powers add: 2.5 D + (−2.0 D) = +0.5 D, and the focal length is 1/0.5 = +2.0 metres.
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