A myopic adult has a far point at `0.1 m`. His power of accomodation is `4` diopters.
(i) What power lenses are required to see distant objects ?
(ii) What is his near point without glasses ?
(iii) What is his near point with glasses ? (Take the image distance from the lens of the eye to the retina to be 2 cm).

Explanation

(i) Image distance, `v=2cm = 0.02m`
Object distance, `u=-0.1m" (far point)"`
Power of mypoic eye vision is
`P_(f)=(1)/(v)-(1)/(u)`
`=(1)/(0.02)-(1)/((-0.1))=(100)/(2)+10=60D`
By using corrective lenses, the far point will lie at infinity. So, power of the eye will be
`P._(f)=(1)/(0.02)-(1)/(oo)=(100)/(2)=50D`
The power of the corrective lens will be
`P_(2)=P._(f)- P_(f)=50-60=-10D`
(ii) Given, power of accommodation = 4D
Let P. be the power of the eye lens for near point, then
`P.=P_(f)=4`
`P.-60=4`
`rArr" "P.=64D`
Let a be the near point without glasses.
`"Then, "64=(1)/(a)+(1)/(0.02)`