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Question
Two lenses A and B have power of (i) +2D and (ii) −4D respectively. What is the nature and focal length of each lens?
Solution
Power of lens A = +2D
Power of lens B = -4D
Power =`1/focal length`
or focal length= `1/power`
Thus, focal length (fA ) of A =`1/2=0.5m =50 cm `
Focal length (fB ) of B = `1/-4=-0.25m=-25 cm `
Therefore, lens A is a convex lens as it has a positive focal length and lens B is concave as it has a negative focal length.
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