Question

At age 16, a girl requires eye glasses with lenses of 2 dioptres in order to read a book at 25 cm. At the age of 32, she finds that while wearing these glasses she must hold a book 40 cm away from the eyes. What power lenses does she require at this age to read a book at 25 cm –

• A. 2.5 dioptres
• B. 3 dioptres
• C. 3.5 dioptres
• D. 4 dioptres

Answer 1

Answer: (C)

3.5 dioptres

Answer 2

= $\frac { 1 } { 50 }$ Ž v = – 200 cm

= $\frac { 1 } { - 200 }$– $\frac { 1 } { - 25 }$ = $\frac { 7 } { 200 }$

P = = 3.5 dioptre