Question

A person has a far point of 17 cm .

part a. What power glasses would correct this vision if the glasses were placed 2.0 cm from the eye?

part b. What power contact lenses, placed on the eye, would the person need?

Answer 1

Far Point of person's eye, f= 17 cm

A). If the glasses were placed 2cm from the eye, then image distance will become

d_i= -(17-2)cm= -15cm= -0.15m (Since image will be on same side of object)

Object distance, d_o= infinity.

Let fonal length of lens be "f", then using Lens Equation,

1/f= (1/d_o) +(1/d_i)

using given values in above,

1/f= 0+(1/-(0.17))

or f= -0.15 m

Then Power of the lens will be

P= 1/f= 1/(-0.15)= -6.67 D (ANS)

B). If contact lenses are placed on the eye, then image distance will become

d_i= -17 cm=-0.19m (Since image will be on same side of object)

Object distance, d_o= infinity.

Let fonal length of contact lenses be f', then using Lens Equation,

1/f'= (1/d_o) +(1/d_i)

using given values in above,

1/f'= 0+(1/-(0.17))

or f'= -0.17m

Then Power of the lens will be

  P'= 1/f'= 1/(-0.17)= -5.88 D (ANS)