A myopia person has been using spectacles of power `- 1.0` dioptre for distant vision. During old age, he also needs to use separate reading glasses of power `+ 2.0` dioptre. Explain what may have happened.

Explanation

Here,P=-1D
`therefore f=1/P=1/(-1)m=-100cm`
For normal vision, far point at it infinity
`thereforeu=-oo`
Using lens formula,`-1/u+1/v=1/f`
we get, `(-1)/(-oo)+1/v=-1/100`
or `1/v=-1/100`
`therefore` v=-100 cm
Thus virtual object at infinity is produced at 100 cm distance using spectacles.
To view objects at distance 25 cm to 100 cm, the person using ability of accomodation of eye which is partially lost in old age for which he needs another spectacles having power.
P=+2D
`therefore f=1/P=1/2=0.5m=50cm and v=25cm`
Using Eq(i), we get
`-1/u+1/25=1/50`
`therefore u=50 cm`
`therefore` Here near point shift to 50 cm.