Reading glasses on a pharmacy rack are labelled 1.75 D. What is the focal length of these lenses?

1. Optometrists prescribe spectacles and contact lenses in diopters, and power is the inverse of the focal length. 2. So f = 1 divided by P. 3. Substituting, f = 1 divided by 1.75 D = 0.571 m. 4. Converting to centimetres gives f = 57.1 cm. 5. The positive sign confirms a converging lens, which is what reading glasses use. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.6 Image Formation by Lenses_