As the person is using spectacles of power `- 1.0 dioptre` (i.e., focal length `-100 cm`), the far point of the person is at `100 cm`. Near point of the eye might have been normal (i.e., 25 cm).
The objects at infinity produce virtual images at `100 cm` (using spectacles). To see objects between `25 cm` and `100 cm`, the person uses the ability of accommodation of his eye lens. This ability is partially lost in old age. The near point of the eye may recede to `50 cm`. He has, therefore to use glasses of suitable power for reading.
Here, `u = -25 cm, v = -50 cm, f = ?`
As `(1)/(v)-(1)/(u)=(1)/(f) :. (-1)/(50)+(1)/(25)=(1)/(f)` or `(-1 + 2)/(50) = (1)/(f)` or `f = 50 cm`
As `P = (100)/(f)=(100)/(50) = + 2 dioptre`.
