Explanation
When a person cannot see far situated object we assume object distance to infinity.
Then image distance be v = -50cm
$$u=\infty \\ v=-50cm$$
then,
$$\dfrac { 1 }{ f } =\dfrac { 1 }{ v } -\dfrac { 1 }{ u } \\ \dfrac { 1 }{ f } =-\dfrac { 1 }{ 50 } \\ f=-50cm=-0.5m$$ (concave lens)
$$power=\dfrac { 1 }{ f } =\dfrac { 1 }{ 0.5 } =-2D$$
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