Two lenses *A* and *B* have power of (*i*) +2D and (*ii*) −4D respectively. What is the nature and focal length of each lens?

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#### Solution

Power of lens A = +2D

Power of lens B = -4D

Power =`1/focal length`

or focal length= `1/power`

Thus, focal length (*f*_{A} ) of A =`1/2=0.5m =50 cm `

Focal length (*f*_{B }) 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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