Optics with corrective eyeglasses

[PhucMa]

Homework Statement

Without glasses, your uncle Albert can see things clearly only if they are between 24.2 cm and 185 cm from his eyes. What power eyeglass lens will correct your uncle's myopia? Assume the lenses will sit 1.66 cm from his eyes.

Homework Equations

1/do + 1/di = 1/f

The Attempt at a Solution

I got the answer right but I'm not sure why it was right. If someone can explain this for me, it would be greatly appreciated. Thanks.

 

[member 553083]

Answer: -4.45 diopters (D) 1/24.2 + 1/185 = 1/f 1/f = 0.010978836 f = 90.6834 1/90.6834 + 1/d = 1/1.66 1/d = 4.45 d = -4.45 D